@@                 Coverage Diff                 @@
##           release/2025-03-17    #1932   +/-   ##
===================================================
  Coverage               71.20%   71.20%           
===================================================
  Files                      86       86           
  Lines                    6998     6998           
===================================================
  Hits                     4983     4983           
  Misses                   2015     2015           

Index: auto-sizes.php
===================================================================
--- auto-sizes.php	(revision 3257264)
+++ auto-sizes.php	(working copy)
@@ -15,10 +15,11 @@
  * @package auto-sizes
  */
 
-// Exit if accessed directly.
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
-	exit;
+	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 // Define the constant.
 if ( defined( 'IMAGE_AUTO_SIZES_VERSION' ) ) {
Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -6,9 +6,11 @@
  * @since 1.0.0
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Displays the HTML generator tag for the plugin.

Index: helper.php
===================================================================
--- helper.php	(revision 3257264)
+++ helper.php	(working copy)
@@ -17,10 +17,10 @@
  */
 function dominant_color_set_image_editors( array $editors ): array {
 	if ( ! class_exists( 'Dominant_Color_Image_Editor_GD' ) ) {
-		require_once __DIR__ . '/class-dominant-color-image-editor-gd.php';
+		require_once __DIR__ . '/class-dominant-color-image-editor-gd.php';// @codeCoverageIgnore
 	}
 	if ( ! class_exists( 'Dominant_Color_Image_Editor_Imagick' ) ) {
-		require_once __DIR__ . '/class-dominant-color-image-editor-imagick.php';
+		require_once __DIR__ . '/class-dominant-color-image-editor-imagick.php';// @codeCoverageIgnore
 	}
 
 	$replaces = array(
Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -7,9 +7,11 @@
  * @since 1.0.0
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Add the dominant color metadata to the attachment.
@@ -63,16 +65,17 @@
 		$attr['data-has-transparency'] = $image_meta['has_transparency'] ? 'true' : 'false';
 
 		$class = $image_meta['has_transparency'] ? 'has-transparency' : 'not-transparent';
-		if ( empty( $attr['class'] ) ) {
+
+		if ( isset( $attr['class'] ) && is_string( $attr['class'] ) && '' !== $attr['class'] ) {
+			$attr['class'] .= ' ' . $class;
+		} else {
 			$attr['class'] = $class;
-		} else {
-			$attr['class'] .= ' ' . $class;
 		}
 	}
 
-	if ( ! empty( $image_meta['dominant_color'] ) ) {
+	if ( isset( $image_meta['dominant_color'] ) && is_string( $image_meta['dominant_color'] ) && '' !== $image_meta['dominant_color'] ) {
 		$attr['data-dominant-color'] = esc_attr( $image_meta['dominant_color'] );
-		$style_attribute             = empty( $attr['style'] ) ? '' : $attr['style'];
+		$style_attribute             = isset( $attr['style'] ) && is_string( $attr['style'] ) ? $attr['style'] : '';
 		$attr['style']               = '--dominant-color: #' . esc_attr( $image_meta['dominant_color'] ) . ';' . $style_attribute;
 	}
 
@@ -138,7 +141,7 @@
 		return $filtered_image;
 	}
 
-	if ( ! empty( $image_meta['dominant_color'] ) ) {
+	if ( isset( $image_meta['dominant_color'] ) && is_string( $image_meta['dominant_color'] ) && '' !== $image_meta['dominant_color'] ) {
 		$processor->set_attribute( 'data-dominant-color', $image_meta['dominant_color'] );
 
 		$style_attribute = '--dominant-color: #' . $image_meta['dominant_color'] . '; ';
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -15,11 +15,12 @@
  * @package dominant-color-images
  */
 
-// Exit if accessed directly.
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
-	exit;
+	exit; // Exit if accessed directly.
 }
 
+
 // Define required constants.
 if ( defined( 'DOMINANT_COLOR_IMAGES_VERSION' ) ) {
 	return;
@@ -29,3 +30,4 @@
 
 require_once __DIR__ . '/helper.php';
 require_once __DIR__ . '/hooks.php';
+// @codeCoverageIgnoreEnd

Index: detect.js
===================================================================
--- detect.js	(revision 3257264)
+++ detect.js	(working copy)
@@ -6,7 +6,7 @@
  * when it is submitted for storage.
  */
 
-const consoleLogPrefix = '[Embed Optimizer]';
+export const name = 'Embed Optimizer';
 
 /**
  * @typedef {import("../optimization-detective/types.ts").URLMetric} URLMetric
@@ -16,29 +16,10 @@
  * @typedef {import("../optimization-detective/types.ts").FinalizeArgs} FinalizeArgs
  * @typedef {import("../optimization-detective/types.ts").FinalizeCallback} FinalizeCallback
  * @typedef {import("../optimization-detective/types.ts").ExtendedElementData} ExtendedElementData
+ * @typedef {import("../optimization-detective/types.ts").LogFunction} LogFunction
  */
 
 /**
- * Logs a message.
- *
- * @param {...*} message
- */
-function log( ...message ) {
-	// eslint-disable-next-line no-console
-	console.log( consoleLogPrefix, ...message );
-}
-
-/**
- * Logs an error.
- *
- * @param {...*} message
- */
-function error( ...message ) {
-	// eslint-disable-next-line no-console
-	console.error( consoleLogPrefix, ...message );
-}
-
-/**
  * Embed element heights.
  *
  * @type {Map<string, DOMRectReadOnly>}
@@ -51,7 +32,10 @@
  * @type {InitializeCallback}
  * @param {InitializeArgs} args Args.
  */
-export async function initialize( { isDebug } ) {
+export async function initialize( { log: _log } ) {
+	// eslint-disable-next-line no-console
+	const log = _log || console.log; // TODO: Remove once Optimization Detective likely updated, or when strict version requirement added in od_init action.
+
 	/** @type NodeListOf<HTMLDivElement> */
 	const embedWrappers = document.querySelectorAll(
 		'.wp-block-embed > .wp-block-embed__wrapper[data-od-xpath]'
@@ -58,12 +42,10 @@
 	);
 
 	for ( /** @type {HTMLElement} */ const embedWrapper of embedWrappers ) {
-		monitorEmbedWrapperForResizes( embedWrapper, isDebug );
+		monitorEmbedWrapperForResizes( embedWrapper, log );
 	}
 
-	if ( isDebug ) {
-		log( 'Loaded embed content rects:', loadedElementContentRects );
-	}
+	log( 'Loaded embed content rects:', loadedElementContentRects );
 }
 
 /**
@@ -73,24 +55,29 @@
  * @param {FinalizeArgs} args Args.
  */
 export async function finalize( {
-	isDebug,
+	log: _log,
+	error: _error,
 	getElementData,
 	extendElementData,
 } ) {
+	/* eslint-disable no-console */
+	// TODO: Remove once Optimization Detective likely updated, or when strict version requirement added in od_init action.
+	const log = _log || console.log;
+	const error = _error || console.error;
+	/* eslint-enable no-console */
+
 	for ( const [ xpath, domRect ] of loadedElementContentRects.entries() ) {
 		try {
 			extendElementData( xpath, {
 				resizedBoundingClientRect: domRect,
 			} );
-			if ( isDebug ) {
-				const elementData = getElementData( xpath );
-				log(
-					`boundingClientRect for ${ xpath } resized:`,
-					elementData.boundingClientRect,
-					'=>',
-					domRect
-				);
-			}
+			const elementData = getElementData( xpath );
+			log(
+				`boundingClientRect for ${ xpath } resized:`,
+				elementData.boundingClientRect,
+				'=>',
+				domRect
+			);
 		} catch ( err ) {
 			error(
 				`Failed to extend element data for ${ xpath } with resizedBoundingClientRect:`,
@@ -105,9 +92,9 @@
  * Monitors embed wrapper for resizes.
  *
  * @param {HTMLDivElement} embedWrapper Embed wrapper DIV.
- * @param {boolean}        isDebug      Whether debug.
+ * @param {LogFunction}    log          The function to call with log messages.
  */
-function monitorEmbedWrapperForResizes( embedWrapper, isDebug ) {
+function monitorEmbedWrapperForResizes( embedWrapper, log ) {
 	if ( ! ( 'odXpath' in embedWrapper.dataset ) ) {
 		throw new Error( 'Embed wrapper missing data-od-xpath attribute.' );
 	}
@@ -115,9 +102,7 @@
 	const observer = new ResizeObserver( ( entries ) => {
 		const [ entry ] = entries;
 		loadedElementContentRects.set( xpath, entry.contentRect );
-		if ( isDebug ) {
-			log( `Resized element ${ xpath }:`, entry.contentRect );
-		}
+		log( `Resized element ${ xpath }:`, entry.contentRect );
 	} );
 	observer.observe( embedWrapper, { box: 'content-box' } );
 }
Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -187,6 +187,11 @@
 	$trigger_error  = static function ( string $message ) use ( $function_name ): void {
 		wp_trigger_error( $function_name, esc_html( $message ) );
 	};
+
+	// As of 1.0.0-beta3, next_tag() allows $query and is beginning to migrate to skip tag closers by default.
+	// In versions prior to this, the method always visited closers and passing a $query actually threw an exception.
+	$tag_query = ! defined( 'OPTIMIZATION_DETECTIVE_VERSION' ) || version_compare( OPTIMIZATION_DETECTIVE_VERSION, '1.0.0-beta3', '>=' )
+		? array( 'tag_closers' => 'visit' ) : null;
 	try {
 		/*
 		 * Determine how to lazy load the embed.
@@ -253,7 +258,7 @@
 					}
 				}
 			}
-		} while ( $html_processor->next_tag() );
+		} while ( $html_processor->next_tag( $tag_query ) );
 		// If there was only one non-inline script, make it lazy.
 		if ( 1 === $script_count && ! $has_inline_script && $html_processor->has_bookmark( $bookmark_names['script'] ) ) {
 			$needs_lazy_script = true;
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -5,7 +5,7 @@
  * Description: Optimizes the performance of embeds through lazy-loading, preconnecting, and reserving space to reduce layout shifts.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 1.0.0-beta1
+ * Version: 1.0.0-beta2
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -71,7 +71,7 @@
 	}
 )(
 	'embed_optimizer_pending_plugin',
-	'1.0.0-beta1',
+	'1.0.0-beta2',
 	static function ( string $version ): void {
 		if ( defined( 'EMBED_OPTIMIZER_VERSION' ) ) {
 			return;
Index: readme.txt
===================================================================
--- readme.txt	(revision 3257264)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   1.0.0-beta1
+Stable tag:   1.0.0-beta2
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, embeds
@@ -67,6 +67,13 @@
 
 == Changelog ==
 
+= 1.0.0-beta2 =
+
+**Enhancements**
+
+* Update `OD_HTML_Tag_Processor::next_tag()` to allow `$query` arg and prepare to skip visiting tag closers by default. ([1872](https://github.com/WordPress/performance/pull/1872))
+* Expose the logging functions to client-side extensions and automatically account for the value of `isDebug`. ([1895](https://github.com/WordPress/performance/pull/1895))
+
 = 1.0.0-beta1 =
 
 **Enhancements**

Index: class-image-prioritizer-background-image-styled-tag-visitor.php
===================================================================
--- class-image-prioritizer-background-image-styled-tag-visitor.php	(revision 3257264)
+++ class-image-prioritizer-background-image-styled-tag-visitor.php	(working copy)
@@ -143,6 +143,7 @@
 	private function maybe_preload_external_lcp_background_image( OD_Tag_Visitor_Context $context ): void {
 		// Gather the tuples of URL Metric group and the common LCP element external background image.
 		// Note the groups of URL Metrics do not change across invocations, we just need to compute this once for all.
+		// TODO: Instead of populating this here, it could be done once per invocation during the od_start_template_optimization action since the page's OD_URL_Metric_Group_Collection is available there.
 		if ( ! is_array( $this->group_common_lcp_element_external_background_images ) ) {
 			$this->group_common_lcp_element_external_background_images = array();
 			foreach ( $context->url_metric_group_collection as $group ) {
Index: class-image-prioritizer-img-tag-visitor.php
===================================================================
--- class-image-prioritizer-img-tag-visitor.php	(revision 3257264)
+++ class-image-prioritizer-img-tag-visitor.php	(working copy)
@@ -226,7 +226,11 @@
 		$crossorigin    = null;
 
 		// Loop through child tags until we reach the closing PICTURE tag.
-		while ( $processor->next_tag() ) {
+		// As of 1.0.0-beta3, next_tag() allows $query and is beginning to migrate to skip tag closers by default.
+		// In versions prior to this, the method always visited closers and passing a $query actually threw an exception.
+		$tag_query = version_compare( OPTIMIZATION_DETECTIVE_VERSION, '1.0.0-beta3', '>=' )
+			? array( 'tag_closers' => 'visit' ) : null;
+		while ( $processor->next_tag( $tag_query ) ) {
 			$tag = $processor->get_tag();
 
 			// If we reached the closing PICTURE tag, break.
Index: detect.js
===================================================================
--- detect.js	(revision 3257264)
+++ detect.js	(working copy)
@@ -9,7 +9,7 @@
  * document has a tag with the same name, ID, and class.
  */
 
-const consoleLogPrefix = '[Image Prioritizer]';
+export const name = 'Image Prioritizer';
 
 /**
  * Detected LCP external background image candidates.
@@ -29,21 +29,10 @@
  * @typedef {import("../optimization-detective/types.ts").InitializeArgs} InitializeArgs
  * @typedef {import("../optimization-detective/types.ts").FinalizeArgs} FinalizeArgs
  * @typedef {import("../optimization-detective/types.ts").FinalizeCallback} FinalizeCallback
+ * @typedef {import("../optimization-detective/types.ts").LogFunction} LogFunction
  */
 
 /**
- * Logs a message.
- *
- * @since 0.3.0
- *
- * @param {...*} message
- */
-function log( ...message ) {
-	// eslint-disable-next-line no-console
-	console.log( consoleLogPrefix, ...message );
-}
-
-/**
  * Initializes extension.
  *
  * @since 0.3.0
@@ -51,10 +40,13 @@
  * @type {InitializeCallback}
  * @param {InitializeArgs} args Args.
  */
-export async function initialize( { isDebug, onLCP } ) {
+export async function initialize( { log: _log, onLCP } ) {
+	// eslint-disable-next-line no-console
+	const log = _log || console.log; // TODO: Remove once Optimization Detective likely updated, or when strict version requirement added in od_init action.
+
 	onLCP(
 		( metric ) => {
-			handleLCPMetric( metric, isDebug );
+			handleLCPMetric( metric, log );
 		},
 		{
 			// This avoids needing to click to finalize LCP candidate. While this is helpful for testing, it also
@@ -70,10 +62,10 @@
  *
  * @since 0.3.0
  *
- * @param {LCPMetric} metric  - LCP Metric.
- * @param {boolean}   isDebug - Whether in debug mode.
+ * @param {LCPMetric}   metric - LCP Metric.
+ * @param {LogFunction} log    - The function to call with log messages.
  */
-function handleLCPMetric( metric, isDebug ) {
+function handleLCPMetric( metric, log ) {
 	for ( const entry of metric.entries ) {
 		// Look only for LCP entries that have a URL and a corresponding element which is not an IMG or VIDEO.
 		if (
@@ -98,19 +90,13 @@
 
 		// Skip URLs that are excessively long. This is the maxLength defined in image_prioritizer_add_element_item_schema_properties().
 		if ( entry.url.length > 500 ) {
-			if ( isDebug ) {
-				log( `Skipping very long URL: ${ entry.url }` );
-			}
+			log( `Skipping very long URL: ${ entry.url }` );
 			return;
 		}
 
 		// Also skip Custom Elements which have excessively long tag names. This is the maxLength defined in image_prioritizer_add_element_item_schema_properties().
 		if ( entry.element.tagName.length > 100 ) {
-			if ( isDebug ) {
-				log(
-					`Skipping very long tag name: ${ entry.element.tagName }`
-				);
-			}
+			log( `Skipping very long tag name: ${ entry.element.tagName }` );
 			return;
 		}
 
@@ -118,16 +104,12 @@
 		// The maxLengths are defined in image_prioritizer_add_element_item_schema_properties().
 		const id = entry.element.getAttribute( 'id' );
 		if ( typeof id === 'string' && id.length > 100 ) {
-			if ( isDebug ) {
-				log( `Skipping very long ID: ${ id }` );
-			}
+			log( `Skipping very long ID: ${ id }` );
 			return;
 		}
 		const className = entry.element.getAttribute( 'class' );
 		if ( typeof className === 'string' && className.length > 500 ) {
-			if ( isDebug ) {
-				log( `Skipping very long className: ${ className }` );
-			}
+			log( `Skipping very long className: ${ className }` );
 			return;
 		}
 
@@ -141,12 +123,10 @@
 			class: className,
 		};
 
-		if ( isDebug ) {
-			log(
-				'Detected external LCP background image:',
-				externalBackgroundImage
-			);
-		}
+		log(
+			'Detected external LCP background image:',
+			externalBackgroundImage
+		);
 
 		externalBackgroundImages.push( externalBackgroundImage );
 	}
@@ -160,7 +140,10 @@
  * @type {FinalizeCallback}
  * @param {FinalizeArgs} args Args.
  */
-export async function finalize( { extendRootData, isDebug } ) {
+export async function finalize( { extendRootData, log: _log } ) {
+	// eslint-disable-next-line no-console
+	const log = _log || console.log; // TODO: Remove once Optimization Detective likely updated, or when strict version requirement added in od_init action.
+
 	if ( externalBackgroundImages.length === 0 ) {
 		return;
 	}
@@ -168,12 +151,10 @@
 	// Get the last detected external background image which is going to be for the LCP element (or very likely will be).
 	const lcpElementExternalBackgroundImage = externalBackgroundImages.pop();
 
-	if ( isDebug ) {
-		log(
-			'Sending external background image for LCP element:',
-			lcpElementExternalBackgroundImage
-		);
-	}
+	log(
+		'Sending external background image for LCP element:',
+		lcpElementExternalBackgroundImage
+	);
 
 	extendRootData( { lcpElementExternalBackgroundImage } );
 }
Index: helper.php
===================================================================
--- helper.php	(revision 3257264)
+++ helper.php	(working copy)
@@ -292,13 +292,21 @@
  *
  * @return WP_REST_Response|WP_HTTP_Response|WP_Error|mixed Result to send to the client.
  * @noinspection PhpDocMissingThrowsInspection
+ * @noinspection PhpDeprecationInspection
  */
 function image_prioritizer_filter_rest_request_before_callbacks( $response, array $handler, WP_REST_Request $request ) {
+	unset( $handler ); // Unused.
+
+	// Check for class existence and use constant or class method calls accordingly.
+	$route_endpoint = class_exists( 'OD_REST_URL_Metrics_Store_Endpoint' )
+						? OD_REST_URL_Metrics_Store_Endpoint::ROUTE_NAMESPACE . OD_REST_URL_Metrics_Store_Endpoint::ROUTE_BASE
+						: OD_REST_API_NAMESPACE . OD_URL_METRICS_ROUTE; // @phpstan-ignore constant.deprecated, constant.deprecated (To be replaced with class method calls in subsequent release.)
+
 	if (
 		$request->get_method() !== 'POST'
 		||
 		// The strtolower() and outer trim are due to \WP_REST_Server::match_request_to_handler() using case-insensitive pattern match and using '$' instead of '\z'.
-		OD_REST_API_NAMESPACE . OD_URL_METRICS_ROUTE !== rtrim( strtolower( ltrim( $request->get_route(), '/' ) ) )
+		( rtrim( strtolower( ltrim( $request->get_route(), '/' ) ) ) !== $route_endpoint )
 	) {
 		return $response;
 	}
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -6,7 +6,7 @@
  * Requires at least: 6.6
  * Requires PHP: 7.2
  * Requires Plugins: optimization-detective
- * Version: 1.0.0-beta1
+ * Version: 1.0.0-beta2
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -72,7 +72,7 @@
 	}
 )(
 	'image_prioritizer_pending_plugin',
-	'1.0.0-beta1',
+	'1.0.0-beta2',
 	static function ( string $version ): void {
 		if ( defined( 'IMAGE_PRIORITIZER_VERSION' ) ) {
 			return;
Index: readme.txt
===================================================================
--- readme.txt	(revision 3257264)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   1.0.0-beta1
+Stable tag:   1.0.0-beta2
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, optimization, image, lcp, lazy-load
@@ -72,6 +72,18 @@
 
 == Changelog ==
 
+= 1.0.0-beta2 =
+
+**Enhancements**
+
+* Update `OD_HTML_Tag_Processor::next_tag()` to allow `$query` arg and prepare to skip visiting tag closers by default. ([1872](https://github.com/WordPress/performance/pull/1872))
+* Expose the logging functions to client-side extensions and automatically account for the value of `isDebug`. ([1895](https://github.com/WordPress/performance/pull/1895))
+
+**Bug Fixes**
+
+* Fix URL encoding in Link HTTP response header. ([1907](https://github.com/WordPress/performance/pull/1907))
+* Fix unpredictable LCP element being identified in a URL Metric Group. ([1903](https://github.com/WordPress/performance/pull/1903))
+
 = 1.0.0-beta1 =
 
 **Enhancements**

Index: class-od-element.php
===================================================================
--- class-od-element.php	(revision 3257264)
+++ class-od-element.php	(working copy)
@@ -33,15 +33,6 @@
 	protected $data;
 
 	/**
-	 * Transitional XPath.
-	 *
-	 * @since 1.0.0
-	 * @todo Remove logic related to transitional_xpath in a subsequent release once URL Metrics have been collected with the new format.
-	 * @var non-empty-string|null
-	 */
-	protected $transitional_xpath = null;
-
-	/**
 	 * URL Metric that this element belongs to.
 	 *
 	 * @since 0.7.0
@@ -98,9 +89,6 @@
 	 * @return mixed|null The property value, or null if not set.
 	 */
 	public function get( string $key ) {
-		if ( 'xpath' === $key ) {
-			return $this->get_xpath();
-		}
 		return $this->data[ $key ] ?? null;
 	}
 
@@ -130,59 +118,11 @@
 	 * Gets XPath for element.
 	 *
 	 * @since 0.7.0
-	 * @since 1.0.0 Returns the transitional XPath format. To access the underlying raw XPath, access the 'xpath' key of the jsonSerialize response.
-	 * @todo Remove logic related to transitional_xpath in a subsequent release once URL Metrics have been collected with the new format.
 	 *
 	 * @return non-empty-string XPath.
 	 */
 	public function get_xpath(): string {
-
-		if ( ! isset( $this->transitional_xpath ) ) {
-			$replacements = array(
-
-				/*
-				 * Convert the original XPath format for elements in the BODY.
-				 *
-				 * Example:
-				 *   /*[1][self::HTML]/*[2][self::BODY]/*[1][self::DIV]/*[1][self::IMG]
-				 *   =>
-				 *   /HTML/BODY/DIV/*[1][self::IMG]
-				 */
-				'#^/\*\[1]\[self::HTML]/\*\[2]\[self::BODY]/\*\[\d+]\[self::([a-zA-Z0-9:_-]+)]#' => '/HTML/BODY/$1',
-
-				/*
-				 * Convert the original XPath format for elements in the HEAD.
-				 *
-				 * Example:
-				 *   /*[1][self::HTML]/*[1][self::HEAD]/*[1][self::META]
-				 *   =>
-				 *   /HTML/HEAD/*[1][self::META]
-				 */
-				'#^/\*\[1\]\[self::HTML\]/\*\[1\]\[self::HEAD]#' => '/HTML/HEAD',
-
-				/*
-				 * Convert the new XPath format for elements in the BODY.
-				 *
-				 * Note that the new XPath format for elements in the HEAD does not need to be converted to the
-				 * transitional format since disambiguating attributes are not used in the HEAD.
-				 *
-				 * Example:
-				 *   /HTML/BODY/DIV[@id='page']/*[1][self::IMG]
-				 *   =>
-				 *   /HTML/BODY/DIV/*[1][self::IMG]
-				 */
-				'#^(/HTML/BODY/\w+)\[@[^\]]+?]#' => '$1',
-			);
-			foreach ( $replacements as $search => $replace ) {
-				$xpath = preg_replace( $search, $replace, $this->data['xpath'], -1, $count );
-				if ( $count > 0 ) {
-					$this->transitional_xpath = $xpath;
-					break;
-				}
-			}
-		}
-
-		return $this->transitional_xpath ?? $this->data['xpath'];
+		return $this->data['xpath'];
 	}
 
 	/**
@@ -249,9 +189,6 @@
 	 */
 	#[ReturnTypeWillChange]
 	public function offsetGet( $offset ) {
-		if ( 'xpath' === $offset ) {
-			return $this->get_xpath();
-		}
 		return $this->data[ $offset ] ?? null;
 	}
 
Index: class-od-html-tag-processor.php
===================================================================
--- class-od-html-tag-processor.php	(revision 3257264)
+++ class-od-html-tag-processor.php	(working copy)
@@ -185,19 +185,6 @@
 	private $bookmarked_open_stacks = array();
 
 	/**
-	 * Stored XPath for the current tag.
-	 *
-	 * This is used so that repeated calls to {@see self::get_stored_xpath()} won't needlessly reconstruct the string.
-	 * This gets cleared whenever {@see self::open_tags()} iterates to the next tag.
-	 *
-	 * @todo Remove this once the XPath transitional period is over.
-	 *
-	 * @since 0.4.0
-	 * @var string|null
-	 */
-	private $current_stored_xpath = null;
-
-	/**
 	 * (Transitional) XPath for the current tag.
 	 *
 	 * This is used to store the old XPath format in a transitional period until which new URL Metrics are expected to
@@ -246,43 +233,40 @@
 	/**
 	 * Finds the next tag.
 	 *
-	 * Unlike the base class, this subclass disallows querying. This is to ensure the breadcrumbs can be tracked.
-	 * It will _always_ visit tag closers.
+	 * Unlike the base class, this subclass currently visits tag closers by default.
+	 * However, for the 1.0.0 release this method will behave the same as the method in
+	 * the base class, where it skips tag closers by default.
 	 *
 	 * @inheritDoc
 	 * @since 0.4.0
+	 * @since 1.0.0 Passing a $query is now allowed. In the 1.0.0 release, this will default to skipping tag closers.
 	 *
-	 * @param array{tag_name?: string|null, match_offset?: int|null, class_name?: string|null, tag_closers?: string|null}|null $query Query, but only null is accepted for this subclass.
+	 * @param array{tag_name?: string|null, match_offset?: int|null, class_name?: string|null, tag_closers?: string|null}|null $query Query.
 	 * @return bool Whether a tag was matched.
-	 *
-	 * @throws InvalidArgumentException If attempting to pass a query.
 	 */
 	public function next_tag( $query = null ): bool {
-		if ( null !== $query ) {
-			throw new InvalidArgumentException( esc_html__( 'Processor subclass does not support queries.', 'optimization-detective' ) );
+		if ( null === $query ) {
+			$query = array( 'tag_closers' => 'visit' );
+			$this->warn(
+				__METHOD__,
+				esc_html__( 'Previously this method always visited tag closers and did not allow a query to be supplied. Now, however, a query can be supplied. To align this method with the behavior of the base class, a future version of this method will default to skipping tag closers.', 'optimization-detective' )
+			);
 		}
-
-		// Elements in the Admin Bar are not relevant for optimization, so this loop ensures that no tags in the Admin Bar are visited.
-		do {
-			$matched = parent::next_tag( array( 'tag_closers' => 'visit' ) );
-		} while ( $matched && $this->is_admin_bar() );
-		return $matched;
+		return parent::next_tag( $query );
 	}
 
 	/**
 	 * Finds the next open tag.
 	 *
+	 * This method will soon be equivalent to calling {@see self::next_tag()} without passing any `$query`.
+	 *
 	 * @since 0.4.0
+	 * @deprecated n.e.x.t Use {@see self::next_tag()} instead.
 	 *
 	 * @return bool Whether a tag was matched.
 	 */
 	public function next_open_tag(): bool {
-		while ( $this->next_tag() ) {
-			if ( ! $this->is_tag_closer() ) {
-				return true;
-			}
-		}
-		return false;
+		return $this->next_tag( array( 'tag_closers' => 'skip' ) );
 	}
 
 	/**
@@ -318,8 +302,7 @@
 	 * @return bool Whether a token was parsed.
 	 */
 	public function next_token(): bool {
-		$this->current_stored_xpath = null; // Clear cache.
-		$this->current_xpath        = null; // Clear cache.
+		$this->current_xpath = null; // Clear cache.
 		++$this->cursor_move_count;
 		if ( ! parent::next_token() ) {
 			$this->open_stack_tags       = array();
@@ -653,23 +636,25 @@
 	 * This predicate will be included once the transitional period is over.
 	 *
 	 * @since 0.4.0
-	 * @todo Replace the logic herein with what is in get_stored_xpath() once the transitional period is over.
 	 *
 	 * @return string XPath.
 	 */
 	public function get_xpath(): string {
-		/*
-		 * This transitional format is used by default for all extensions. The non-transitional format is used only in
-		 * od_optimize_template_output_buffer() when setting the data-od-xpath attribute. This is so that the new format
-		 * will replace the old format as new URL Metrics are collected. After a month of the new format being live, the
-		 * transitional format can be eliminated. See the corresponding logic in OD_Element for normalizing both the
-		 * old and new XPath formats to use the transitional format.
-		 */
 		if ( null === $this->current_xpath ) {
 			$this->current_xpath = '';
 			foreach ( $this->get_indexed_breadcrumbs() as $i => list( $tag_name, $index, $attributes ) ) {
-				if ( $i < 2 || ( 2 === $i && '/HTML/BODY' === $this->current_xpath ) ) {
+				if ( $i < 2 ) {
 					$this->current_xpath .= "/$tag_name";
+				} elseif ( 2 === $i && '/HTML/BODY' === $this->current_xpath ) {
+					$segment = "/$tag_name";
+					foreach ( $attributes as $attribute_name => $attribute_value ) {
+						$segment .= sprintf(
+							"[@%s='%s']",
+							$attribute_name,
+							$attribute_value // Note: $attribute_value has already been validated to only contain safe characters /^[a-zA-Z0-9_.\s:-]*/ which do not need escaping.
+						);
+					}
+					$this->current_xpath .= $segment;
 				} else {
 					$this->current_xpath .= sprintf( '/*[%d][self::%s]', $index + 1, $tag_name );
 				}
@@ -681,49 +666,32 @@
 	/**
 	 * Gets stored XPath for the current open tag.
 	 *
-	 * This method is temporary for a transition period while new URL Metrics are collected for active installs. Once
-	 * the transition period is over, the logic in this method can be moved to {@see self::get_xpath()} and this method
-	 * can simply be an alias for that one. See related logic in {@see OD_Element::get_xpath()}. This function is only
-	 * used internally by Optimization Detective in {@see od_optimize_template_output_buffer()}.
+	 * This method was temporary for a transition period while new URL Metrics are collected for active installs
 	 *
 	 * @since 1.0.0
-	 * @todo Move the logic in this method to the get_xpath() method and let this be an alias for that method once the transitional period is over.
 	 * @access private
+	 * @deprecated
+	 * @codeCoverageIgnore
 	 *
 	 * @return string XPath.
 	 */
 	public function get_stored_xpath(): string {
-		if ( null === $this->current_stored_xpath ) {
-			$this->current_stored_xpath = '';
-			foreach ( $this->get_indexed_breadcrumbs() as $i => list( $tag_name, $index, $attributes ) ) {
-				if ( $i < 2 ) {
-					$this->current_stored_xpath .= "/$tag_name";
-				} elseif ( 2 === $i && '/HTML/BODY' === $this->current_stored_xpath ) {
-					$segment = "/$tag_name";
-					foreach ( $attributes as $attribute_name => $attribute_value ) {
-						$segment .= sprintf(
-							"[@%s='%s']",
-							$attribute_name,
-							$attribute_value // Note: $attribute_value has already been validated to only contain safe characters /^[a-zA-Z0-9_.\s:-]*/ which do not need escaping.
-						);
-					}
-					$this->current_stored_xpath .= $segment;
-				} else {
-					$this->current_stored_xpath .= sprintf( '/*[%d][self::%s]', $index + 1, $tag_name );
-				}
-			}
-		}
-		return $this->current_stored_xpath;
+		return $this->get_xpath();
 	}
 
 	/**
 	 * Returns whether the processor is currently at or inside the admin bar.
 	 *
+	 * This is only intended to be used internally by Optimization Detective as part of the "optimization loop". Tag
+	 * visitors should not rely on this method as it may be deprecated in the future, especially with a migration to
+	 * WP_HTML_Processor after {@link https://core.trac.wordpress.org/ticket/63020} is implemented.
+	 *
 	 * @since 1.0.0
+	 * @access private
 	 *
 	 * @return bool Whether at or inside the admin bar.
 	 */
-	private function is_admin_bar(): bool {
+	public function is_admin_bar(): bool {
 		return (
 			isset( $this->open_stack_tags[2], $this->open_stack_attributes[2]['id'] )
 			&&
@@ -734,29 +702,33 @@
 	}
 
 	/**
-	 * Append HTML to the HEAD.
+	 * Appends raw HTML to the HEAD.
 	 *
-	 * The provided HTML must be valid! No validation is performed.
+	 *  The provided HTML must be valid for insertion in the HEAD. No validation is currently performed. However, in the
+	 *  future the HTML Processor may be used to ensure the validity of the provided HTML. At that time, when invalid
+	 *  HTML is provided, this method may emit a `_doing_it_wrong()` warning.
 	 *
 	 * @since 0.4.0
 	 *
-	 * @param string $html HTML to inject.
+	 * @param string $raw_html Raw HTML to inject.
 	 */
-	public function append_head_html( string $html ): void {
-		$this->buffered_text_replacements[ self::END_OF_HEAD_BOOKMARK ][] = $html;
+	public function append_head_html( string $raw_html ): void {
+		$this->buffered_text_replacements[ self::END_OF_HEAD_BOOKMARK ][] = $raw_html;
 	}
 
 	/**
-	 * Append HTML to the BODY.
+	 * Appends raw HTML to the BODY.
 	 *
-	 * The provided HTML must be valid! No validation is performed.
+	 * The provided HTML must be valid for insertion in the BODY. No validation is currently performed. However, in the
+	 * future the HTML Processor may be used to ensure the validity of the provided HTML. At that time, when invalid
+	 * HTML is provided, this method may emit a `_doing_it_wrong()` warning.
 	 *
 	 * @since 0.4.0
 	 *
-	 * @param string $html HTML to inject.
+	 * @param string $raw_html Raw HTML to inject.
 	 */
-	public function append_body_html( string $html ): void {
-		$this->buffered_text_replacements[ self::END_OF_BODY_BOOKMARK ][] = $html;
+	public function append_body_html( string $raw_html ): void {
+		$this->buffered_text_replacements[ self::END_OF_BODY_BOOKMARK ][] = $raw_html;
 	}
 
 	/**
Index: class-od-link-collection.php
===================================================================
--- class-od-link-collection.php	(revision 3257264)
+++ class-od-link-collection.php	(working copy)
@@ -323,21 +323,19 @@
 	/**
 	 * Encodes a URL for serving in an HTTP response header.
 	 *
-	 * @since n.e.x.t
+	 * @since 1.0.0
 	 *
-	 * @param string $url URL to percent encode. Any existing percent encodings will first be decoded.
+	 * @param string $url URL to percent encode.
 	 * @return string Percent-encoded URL.
 	 */
 	private function encode_url_for_response_header( string $url ): string {
-		$decoded_url = urldecode( $url );
-
 		// Encode characters not allowed in a URL per RFC 3986 (anything that is not among the reserved and unreserved characters).
 		$encoded_url = (string) preg_replace_callback(
-			'/[^A-Za-z0-9\-._~:\/?#\[\]@!$&\'()*+,;=]/',
+			'/[^A-Za-z0-9\-._~:\/?#\[\]@!$&\'()*+,;=%]/',
 			static function ( $matches ) {
 				return rawurlencode( $matches[0] );
 			},
-			$decoded_url
+			$url
 		);
 		return esc_url_raw( $encoded_url );
 	}
Index: class-od-tag-visitor-context.php
===================================================================
--- class-od-tag-visitor-context.php	(revision 3257264)
+++ class-od-tag-visitor-context.php	(working copy)
@@ -110,7 +110,7 @@
 	 * @throws Error When property is unknown.
 	 */
 	public function __get( string $name ) {
-		// Note that there is intentionally not a case for 'visited_tag_state'.
+		// Note: There is intentionally not a 'visited_tag_state' case to expose $this->visited_tag_state.
 		switch ( $name ) {
 			case 'processor':
 				return $this->processor;
Index: class-od-url-metric-group-collection.php
===================================================================
--- class-od-url-metric-group-collection.php	(revision 3257264)
+++ class-od-url-metric-group-collection.php	(working copy)
@@ -233,7 +233,7 @@
 	}
 
 	/**
-	 * Gets the first URL Metric group.
+	 * Gets the first URL Metric group (with the lowest minimum viewport width, e.g. for mobile).
 	 *
 	 * This group normally represents viewports for mobile devices. This group always has a minimum viewport width of 0
 	 * and the maximum viewport width corresponds to the smallest defined breakpoint returned by
@@ -248,7 +248,7 @@
 	}
 
 	/**
-	 * Gets the last URL Metric group.
+	 * Gets the last URL Metric group (with the highest minimum viewport width, e.g. for desktop).
 	 *
 	 * This group normally represents viewports for desktop devices.  This group always has a minimum viewport width
 	 * defined as one greater than the largest breakpoint returned by {@see od_get_breakpoint_max_widths()}.
@@ -266,6 +266,7 @@
 	 * Clears result cache.
 	 *
 	 * @since 0.3.0
+	 * @access private
 	 */
 	public function clear_cache(): void {
 		$this->result_cache = array();
@@ -298,6 +299,7 @@
 	 *
 	 * @since 0.1.0
 	 * @throws InvalidArgumentException If there is no group available to add a URL Metric to.
+	 * @access private
 	 *
 	 * @param OD_URL_Metric $new_url_metric New URL Metric.
 	 */
@@ -317,7 +319,7 @@
 	}
 
 	/**
-	 * Gets group for viewport width.
+	 * Gets the group for the provided viewport width.
 	 *
 	 * @since 0.1.0
 	 * @throws InvalidArgumentException When there is no group for the provided viewport width. This would only happen if a negative width is provided.
@@ -411,8 +413,12 @@
 	}
 
 	/**
-	 * Checks whether every group is complete.
+	 * Checks whether every group is complete (full sample of non-stale URL Metrics).
 	 *
+	 * Completeness means the full sample size of URL Metrics has been collected,
+	 * none of the collected URL Metrics are stale (with a mismatching ETag or a
+	 * timestamp older than the freshness TTL).
+	 *
 	 * @since 0.1.0
 	 * @see OD_URL_Metric_Group::is_complete()
 	 *
@@ -438,7 +444,7 @@
 	}
 
 	/**
-	 * Gets the groups with the provided LCP element XPath.
+	 * Gets the groups which have an LCP element with the provided XPath.
 	 *
 	 * @since 0.3.0
 	 * @see OD_URL_Metric_Group::get_lcp_element()
@@ -468,7 +474,7 @@
 	}
 
 	/**
-	 * Gets common LCP element.
+	 * Gets the LCP element which is shared by all groups, or at least the first group (mobile) and last group (desktop) if the intermediary groups are not populated.
 	 *
 	 * @since 0.3.0
 	 * @since 0.9.0 An LCP element is also considered common if it is the same in the narrowest and widest viewport groups, and all intermediate groups are empty.
@@ -585,7 +591,7 @@
 	}
 
 	/**
-	 * Gets all elements' status for whether they are positioned in any initial viewport.
+	 * Gets the status for whether each element is positioned in any initial viewport.
 	 *
 	 * An element is positioned in the initial viewport if its `boundingClientRect.top` is less than the
 	 * `viewport.height` for any of its recorded URL Metrics. Note that even though the element may be positioned in the
Index: class-od-url-metric-group.php
===================================================================
--- class-od-url-metric-group.php	(revision 3257264)
+++ class-od-url-metric-group.php	(working copy)
@@ -242,6 +242,7 @@
 	 * Adds a URL Metric to the group.
 	 *
 	 * @since 0.1.0
+	 * @access private
 	 *
 	 * @throws InvalidArgumentException If the viewport width of the URL Metric is not within the min/max bounds of the group.
 	 *
@@ -279,7 +280,8 @@
 	 * Determines whether the URL Metric group is complete.
 	 *
 	 * A group is complete if it has the full sample size of URL Metrics
-	 * and all of these URL Metrics are fresh.
+	 * and all of these URL Metrics are fresh (with a current ETag and a
+	 * timestamp that is not older than the freshness TTL).
 	 *
 	 * @since 0.1.0
 	 * @since 0.9.0 If the current environment's generated ETag does not match the URL Metric's ETag, the URL Metric is considered stale.
@@ -358,7 +360,20 @@
 			 */
 			$breadcrumb_element = array();
 
-			foreach ( $this->url_metrics as $url_metric ) {
+			// Prefer to use URL Metrics which have a current ETag.
+			$url_metrics = array_filter(
+				$this->url_metrics,
+				function ( OD_URL_Metric $url_metric ): bool {
+					return $url_metric->get_etag() === $this->get_collection()->get_current_etag();
+				}
+			);
+
+			// Otherwise, if no URL Metrics have a current ETag, fall back to using all the stale ones.
+			if ( count( $url_metrics ) === 0 ) {
+				$url_metrics = $this->url_metrics;
+			}
+
+			foreach ( $url_metrics as $url_metric ) {
 				foreach ( $url_metric->get_elements() as $element ) {
 					if ( ! $element->is_lcp() ) {
 						continue;
@@ -486,6 +501,7 @@
 	 * Clears result cache.
 	 *
 	 * @since 0.9.0
+	 * @access private
 	 */
 	public function clear_cache(): void {
 		$this->result_cache = array();
Index: class-od-url-metric.php
===================================================================
--- class-od-url-metric.php	(revision 3257264)
+++ class-od-url-metric.php	(working copy)
@@ -145,11 +145,11 @@
 	}
 
 	/**
-	 * Gets the group that this URL Metric is a part of (which may not be any).
+	 * Gets the group that this URL Metric is a part of.
 	 *
 	 * @since 0.7.0
 	 *
-	 * @return OD_URL_Metric_Group|null Group.
+	 * @return OD_URL_Metric_Group|null Group. Null will never occur in the context of a tag visitor.
 	 */
 	public function get_group(): ?OD_URL_Metric_Group {
 		return $this->group;
@@ -159,6 +159,7 @@
 	 * Sets the group that this URL Metric is a part of.
 	 *
 	 * @since 0.7.0
+	 * @access private
 	 *
 	 * @param OD_URL_Metric_Group $group Group.
 	 *
@@ -177,6 +178,7 @@
 	 * @since 0.1.0
 	 * @since 0.9.0 Added the 'etag' property to the schema.
 	 * @since 1.0.0 The 'etag' property is now required.
+	 * @access private
 	 *
 	 * @todo Cache the return value?
 	 *
Index: detect.js
===================================================================
--- detect.js	(revision 3257264)
+++ detect.js	(working copy)
@@ -1,3 +1,5 @@
+// noinspection JSUnusedGlobalSymbols
+
 /**
  * @typedef {import("web-vitals").LCPMetric} LCPMetric
  * @typedef {import("web-vitals").LCPMetricWithAttribution} LCPMetricWithAttribution
@@ -17,6 +19,7 @@
  * @typedef {import("./types.ts").Extension} Extension
  * @typedef {import("./types.ts").ExtendedRootData} ExtendedRootData
  * @typedef {import("./types.ts").ExtendedElementData} ExtendedElementData
+ * @typedef {import("./types.ts").Logger} Logger
  */
 
 const win = window;
@@ -66,33 +69,60 @@
 }
 
 /**
- * Logs a message.
+ * Creates a logger object with log, warn, and error methods.
  *
- * @param {...*} message
+ * @param {boolean} [debugMode=false] - Whether to enable debug mode.
+ * @param {string}  [prefix='']       - Prefix to prepend to the console message.
+ * @return {Logger} Logger object with log, info, warn, and error methods.
  */
-function log( ...message ) {
-	// eslint-disable-next-line no-console
-	console.log( consoleLogPrefix, ...message );
-}
+function createLogger( debugMode = false, prefix = '' ) {
+	return {
+		/**
+		 * Logs a message if debug mode is enabled.
+		 *
+		 * @param {...*} message - The message(s) to log.
+		 */
+		log( ...message ) {
+			if ( debugMode ) {
+				// eslint-disable-next-line no-console
+				console.log( prefix, ...message );
+			}
+		},
 
-/**
- * Logs a warning.
- *
- * @param {...*} message
- */
-function warn( ...message ) {
-	// eslint-disable-next-line no-console
-	console.warn( consoleLogPrefix, ...message );
-}
+		/**
+		 * Logs an informational message if debug mode is enabled.
+		 *
+		 * @param {...*} message - The message(s) to log as info.
+		 */
+		info( ...message ) {
+			if ( debugMode ) {
+				// eslint-disable-next-line no-console
+				console.info( prefix, ...message );
+			}
+		},
 
-/**
- * Logs an error.
- *
- * @param {...*} message
- */
-function error( ...message ) {
-	// eslint-disable-next-line no-console
-	console.error( consoleLogPrefix, ...message );
+		/**
+		 * Logs a warning if debug mode is enabled.
+		 *
+		 * @param {...*} message - The message(s) to log as a warning.
+		 */
+		warn( ...message ) {
+			if ( debugMode ) {
+				// eslint-disable-next-line no-console
+				console.warn( prefix, ...message );
+			}
+		},
+
+		/**
+		 * Logs an error.
+		 *
+		 * @param {...*} message - The message(s) to log as an error.
+		 */
+		error( ...message ) {
+			// eslint-disable-next-line no-console
+			console.error( prefix, ...message );
+		},
+	};
 }
 
 /**
@@ -126,35 +156,52 @@
  * @param {string}               currentETag          - Current ETag.
  * @param {string}               currentUrl           - Current URL.
  * @param {URLMetricGroupStatus} urlMetricGroupStatus - URL Metric group status.
- * @return {Promise<string>} Session storage key.
+ * @param {Logger}               logger               - Logger.
+ * @return {Promise<string|null>} Session storage key for the current URL or null if crypto is not available or caused an error.
  */
 async function getAlreadySubmittedSessionStorageKey(
 	currentETag,
 	currentUrl,
-	urlMetricGroupStatus
+	urlMetricGroupStatus,
+	{ warn, error }
 ) {
-	const message = [
-		currentETag,
-		currentUrl,
-		urlMetricGroupStatus.minimumViewportWidth,
-		urlMetricGroupStatus.maximumViewportWidth || '',
-	].join( '-' );
+	if ( ! window.crypto || ! window.crypto.subtle ) {
+		warn(
+			'Unable to generate sessionStorage key for already-submitted URL since crypto is not available, likely due to to the page not being served via HTTPS.'
+		);
+		return null;
+	}
 
-	/*
-	 * Note that the components are hashed for a couple of reasons:
-	 *
-	 * 1. It results in a consistent length string devoid of any special characters that could cause problems.
-	 * 2. Since the key includes the URL, hashing it avoids potential privacy concerns where the sessionStorage is
-	 *    examined to see which URLs the client went to.
-	 *
-	 * The SHA-1 algorithm is chosen since it is the fastest and there is no need for cryptographic security.
-	 */
-	const msgBuffer = new TextEncoder().encode( message );
-	const hashBuffer = await crypto.subtle.digest( 'SHA-1', msgBuffer );
-	const hashHex = Array.from( new Uint8Array( hashBuffer ) )
-		.map( ( b ) => b.toString( 16 ).padStart( 2, '0' ) )
-		.join( '' );
-	return `odSubmitted-${ hashHex }`;
+	try {
+		const message = [
+			currentETag,
+			currentUrl,
+			urlMetricGroupStatus.minimumViewportWidth,
+			urlMetricGroupStatus.maximumViewportWidth || '',
+		].join( '-' );
+
+		/*
+		 * Note that the components are hashed for a couple of reasons:
+		 *
+		 * 1. It results in a consistent length string devoid of any special characters that could cause problems.
+		 * 2. Since the key includes the URL, hashing it avoids potential privacy concerns where the sessionStorage is
+		 *    examined to see which URLs the client went to.
+		 *
+		 * The SHA-1 algorithm is chosen since it is the fastest and there is no need for cryptographic security.
+		 */
+		const msgBuffer = new TextEncoder().encode( message );
+		const hashBuffer = await crypto.subtle.digest( 'SHA-1', msgBuffer );
+		const hashHex = Array.from( new Uint8Array( hashBuffer ) )
+			.map( ( b ) => b.toString( 16 ).padStart( 2, '0' ) )
+			.join( '' );
+		return `odSubmitted-${ hashHex }`;
+	} catch ( err ) {
+		error(
+			'Unable to generate sessionStorage key for already-submitted URL due to error:',
+			err
+		);
+		return null;
+	}
 }
 
 /**
@@ -169,7 +216,7 @@
 /**
  * Recursively freezes an object to prevent mutation.
  *
- * @param {Object} obj Object to recursively freeze.
+ * @param {Object} obj - Object to recursively freeze.
  */
 function recursiveFreeze( obj ) {
 	for ( const prop of Object.getOwnPropertyNames( obj ) ) {
@@ -248,7 +295,7 @@
 /**
  * Gets element data.
  *
- * @param {string} xpath XPath.
+ * @param {string} xpath - XPath.
  * @return {ElementData|null} Element data, or null if no element for the XPath exists.
  */
 function getElementData( xpath ) {
@@ -264,8 +311,8 @@
 /**
  * Extends element data.
  *
- * @param {string}              xpath      XPath.
- * @param {ExtendedElementData} properties Properties.
+ * @param {string}              xpath      - XPath.
+ * @param {ExtendedElementData} properties - Properties.
  */
 function extendElementData( xpath, properties ) {
 	if ( ! elementsByXPath.has( xpath ) ) {
@@ -290,23 +337,23 @@
 /**
  * Detects the LCP element, loaded images, client viewport and store for future optimizations.
  *
- * @param {Object}                 args                            Args.
- * @param {string[]}               args.extensionModuleUrls        URLs for extension script modules to import.
- * @param {number}                 args.minViewportAspectRatio     Minimum aspect ratio allowed for the viewport.
- * @param {number}                 args.maxViewportAspectRatio     Maximum aspect ratio allowed for the viewport.
- * @param {boolean}                args.isDebug                    Whether to show debug messages.
- * @param {string}                 args.restApiEndpoint            URL for where to send the detection data.
- * @param {string}                 [args.restApiNonce]             Nonce for the REST API when the user is logged-in.
- * @param {string}                 args.currentETag                Current ETag.
- * @param {string}                 args.currentUrl                 Current URL.
- * @param {string}                 args.urlMetricSlug              Slug for URL Metric.
- * @param {number|null}            args.cachePurgePostId           Cache purge post ID.
- * @param {string}                 args.urlMetricHMAC              HMAC for URL Metric storage.
- * @param {URLMetricGroupStatus[]} args.urlMetricGroupStatuses     URL Metric group statuses.
- * @param {number}                 args.storageLockTTL             The TTL (in seconds) for the URL Metric storage lock.
- * @param {number}                 args.freshnessTTL               The freshness age (TTL) for a given URL Metric.
- * @param {string}                 args.webVitalsLibrarySrc        The URL for the web-vitals library.
- * @param {CollectionDebugData}    [args.urlMetricGroupCollection] URL Metric group collection, when in debug mode.
+ * @param {Object}                 args                            - Args.
+ * @param {string[]}               args.extensionModuleUrls        - URLs for extension script modules to import.
+ * @param {number}                 args.minViewportAspectRatio     - Minimum aspect ratio allowed for the viewport.
+ * @param {number}                 args.maxViewportAspectRatio     - Maximum aspect ratio allowed for the viewport.
+ * @param {boolean}                args.isDebug                    - Whether to show debug messages.
+ * @param {string}                 args.restApiEndpoint            - URL for where to send the detection data.
+ * @param {string}                 [args.restApiNonce]             - Nonce for the REST API when the user is logged-in.
+ * @param {string}                 args.currentETag                - Current ETag.
+ * @param {string}                 args.currentUrl                 - Current URL.
+ * @param {string}                 args.urlMetricSlug              - Slug for URL Metric.
+ * @param {number|null}            args.cachePurgePostId           - Cache purge post ID.
+ * @param {string}                 args.urlMetricHMAC              - HMAC for URL Metric storage.
+ * @param {URLMetricGroupStatus[]} args.urlMetricGroupStatuses     - URL Metric group statuses.
+ * @param {number}                 args.storageLockTTL             - The TTL (in seconds) for the URL Metric storage lock.
+ * @param {number}                 args.freshnessTTL               - The freshness age (TTL) for a given URL Metric.
+ * @param {string}                 args.webVitalsLibrarySrc        - The URL for the web-vitals library.
+ * @param {CollectionDebugData}    [args.urlMetricGroupCollection] - URL Metric group collection, when in debug mode.
  */
 export default async function detect( {
 	minViewportAspectRatio,
@@ -326,6 +373,9 @@
 	webVitalsLibrarySrc,
 	urlMetricGroupCollection,
 } ) {
+	const logger = createLogger( isDebug, consoleLogPrefix );
+	const { log, warn, error } = logger;
+
 	if ( isDebug ) {
 		const allUrlMetrics = /** @type Array<UrlMetricDebugData> */ [];
 		for ( const group of urlMetricGroupCollection.groups ) {
@@ -345,11 +395,9 @@
 	}
 
 	if ( win.innerWidth === 0 || win.innerHeight === 0 ) {
-		if ( isDebug ) {
-			log(
-				'Window must have non-zero dimensions for URL Metric collection.'
-			);
-		}
+		log(
+			'Window must have non-zero dimensions for URL Metric collection.'
+		);
 		return;
 	}
 
@@ -359,9 +407,7 @@
 		urlMetricGroupStatuses
 	);
 	if ( urlMetricGroupStatus.complete ) {
-		if ( isDebug ) {
-			log( 'No need for URL Metrics from the current viewport.' );
-		}
+		log( 'No need for URL Metrics from the current viewport.' );
 		return;
 	}
 
@@ -370,9 +416,13 @@
 		await getAlreadySubmittedSessionStorageKey(
 			currentETag,
 			currentUrl,
-			urlMetricGroupStatus
+			urlMetricGroupStatus,
+			logger
 		);
-	if ( alreadySubmittedSessionStorageKey in sessionStorage ) {
+	if (
+		null !== alreadySubmittedSessionStorageKey &&
+		alreadySubmittedSessionStorageKey in sessionStorage
+	) {
 		const previousVisitTime = parseInt(
 			sessionStorage.getItem( alreadySubmittedSessionStorageKey ),
 			10
@@ -381,12 +431,10 @@
 			! isNaN( previousVisitTime ) &&
 			( getCurrentTime() - previousVisitTime ) / 1000 < freshnessTTL
 		) {
-			if ( isDebug ) {
-				log(
-					'The current client session already submitted a fresh URL Metric for this URL so a new one will not be collected now.'
-				);
-				return;
-			}
+			log(
+				'The current client session already submitted a fresh URL Metric for this URL so a new one will not be collected now.'
+			);
+			return;
 		}
 	}
 
@@ -396,11 +444,9 @@
 		aspectRatio < minViewportAspectRatio ||
 		aspectRatio > maxViewportAspectRatio
 	) {
-		if ( isDebug ) {
-			warn(
-				`Viewport aspect ratio (${ aspectRatio }) is not in the accepted range of ${ minViewportAspectRatio } to ${ maxViewportAspectRatio }.`
-			);
-		}
+		warn(
+			`Viewport aspect ratio (${ aspectRatio }) is not in the accepted range of ${ minViewportAspectRatio } to ${ maxViewportAspectRatio }.`
+		);
 		return;
 	}
 
@@ -434,9 +480,7 @@
 	// od_is_url_metric_storage_locked() function returns true. However, the downside with that is page caching could
 	// result in metrics missed from being gathered when a user navigates around a site and primes the page cache.
 	if ( isStorageLocked( getCurrentTime(), storageLockTTL ) ) {
-		if ( isDebug ) {
-			warn( 'Aborted detection due to storage being locked.' );
-		}
+		warn( 'Aborted detection due to storage being locked.' );
 		return;
 	}
 
@@ -461,17 +505,13 @@
 	// TODO: Does this make sense here?
 	// Prevent detection when page is not scrolled to the initial viewport.
 	if ( doc.documentElement.scrollTop > 0 ) {
-		if ( isDebug ) {
-			warn(
-				'Aborted detection since initial scroll position of page is not at the top.'
-			);
-		}
+		warn(
+			'Aborted detection since initial scroll position of page is not at the top.'
+		);
 		return;
 	}
 
-	if ( isDebug ) {
-		log( 'Proceeding with detection' );
-	}
+	log( 'Proceeding with detection' );
 
 	/** @type {Map<string, Extension>} */
 	const extensions = new Map();
@@ -487,10 +527,19 @@
 			/** @type {Extension} */
 			const extension = await import( extensionModuleUrl );
 			extensions.set( extensionModuleUrl, extension );
+
+			const extensionLogger = createLogger(
+				isDebug,
+				`[Optimization Detective: ${
+					extension.name || 'Unnamed Extension'
+				}]`
+			);
+
 			// TODO: There should to be a way to pass additional args into the module. Perhaps extensionModuleUrls should be a mapping of URLs to args.
 			if ( extension.initialize instanceof Function ) {
 				const initializePromise = extension.initialize( {
 					isDebug,
+					...extensionLogger,
 					onTTFB,
 					onFCP,
 					onLCP,
@@ -607,9 +656,7 @@
 
 	// Stop observing.
 	disconnectIntersectionObserver();
-	if ( isDebug ) {
-		log( 'Detection is stopping.' );
-	}
+	log( 'Detection is stopping.' );
 
 	urlMetric = {
 		url: currentUrl,
@@ -625,9 +672,7 @@
 	for ( const elementIntersection of elementIntersections ) {
 		const xpath = breadcrumbedElementsMap.get( elementIntersection.target );
 		if ( ! xpath ) {
-			if ( isDebug ) {
-				error( 'Unable to look up XPath for element' );
-			}
+			warn( 'Unable to look up XPath for element' );
 			continue;
 		}
 
@@ -657,9 +702,7 @@
 		elementsByXPath.set( elementData.xpath, elementData );
 	}
 
-	if ( isDebug ) {
-		log( 'Current URL Metric:', urlMetric );
-	}
+	log( 'Current URL Metric:', urlMetric );
 
 	// Wait for the page to be hidden.
 	await new Promise( ( resolve ) => {
@@ -680,11 +723,7 @@
 	// Only proceed with submitting the URL Metric if viewport stayed the same size. Changing the viewport size (e.g. due
 	// to resizing a window or changing the orientation of a device) will result in unexpected metrics being collected.
 	if ( didWindowResize ) {
-		if ( isDebug ) {
-			log(
-				'Aborting URL Metric collection due to viewport size change.'
-			);
-		}
+		log( 'Aborting URL Metric collection due to viewport size change.' );
 		return;
 	}
 
@@ -701,9 +740,17 @@
 			extension,
 		] of extensions.entries() ) {
 			if ( extension.finalize instanceof Function ) {
+				const extensionLogger = createLogger(
+					isDebug,
+					`[Optimization Detective: ${
+						extension.name || 'Unnamed Extension'
+					}]`
+				);
+
 				try {
 					const finalizePromise = extension.finalize( {
 						isDebug,
+						...extensionLogger,
 						getRootData,
 						getElementData,
 						extendElementData,
@@ -748,10 +795,10 @@
 	const maxBodyLengthKiB = 64;
 	const maxBodyLengthBytes = maxBodyLengthKiB * 1024;
 
-	// TODO: Consider adding replacer to reduce precision on numbers in DOMRect to reduce payload size.
 	const jsonBody = JSON.stringify( urlMetric );
+	const payloadBlob = new Blob( [ jsonBody ], { type: 'application/json' } );
 	const percentOfBudget =
-		( jsonBody.length / ( maxBodyLengthKiB * 1000 ) ) * 100;
+		( payloadBlob.size / ( maxBodyLengthKiB * 1000 ) ) * 100;
 
 	/*
 	 * According to the fetch() spec:
@@ -759,15 +806,13 @@
 	 * This is what browsers also implement for navigator.sendBeacon(). Therefore, if the size of the JSON is greater
 	 * than the maximum, we should avoid even trying to send it.
 	 */
-	if ( jsonBody.length > maxBodyLengthBytes ) {
-		if ( isDebug ) {
-			error(
-				`Unable to send URL Metric because it is ${ jsonBody.length.toLocaleString() } bytes, ${ Math.round(
-					percentOfBudget
-				) }% of ${ maxBodyLengthKiB } KiB limit:`,
-				urlMetric
-			);
-		}
+	if ( payloadBlob.size > maxBodyLengthBytes ) {
+		error(
+			`Unable to send URL Metric because it is ${ payloadBlob.size.toLocaleString() } bytes, ${ Math.round(
+				percentOfBudget
+			) }% of ${ maxBodyLengthKiB } KiB limit:`,
+			urlMetric
+		);
 		return;
 	}
 
@@ -776,22 +821,22 @@
 	setStorageLock( getCurrentTime() );
 
 	// Remember that the URL Metric was submitted for this URL to avoid having multiple entries submitted by the same client.
-	sessionStorage.setItem(
-		alreadySubmittedSessionStorageKey,
-		String( getCurrentTime() )
-	);
+	if ( null !== alreadySubmittedSessionStorageKey ) {
+		sessionStorage.setItem(
+			alreadySubmittedSessionStorageKey,
+			String( getCurrentTime() )
+		);
+	}
 
-	if ( isDebug ) {
-		const message = `Sending URL Metric (${ jsonBody.length.toLocaleString() } bytes, ${ Math.round(
-			percentOfBudget
-		) }% of ${ maxBodyLengthKiB } KiB limit):`;
+	const message = `Sending URL Metric (${ payloadBlob.size.toLocaleString() } bytes, ${ Math.round(
+		percentOfBudget
+	) }% of ${ maxBodyLengthKiB } KiB limit):`;
 
-		// The threshold of 50% is used because the limit for all beacons combined is 64 KiB, not just the data for one beacon.
-		if ( percentOfBudget < 50 ) {
-			log( message, urlMetric );
-		} else {
-			warn( message, urlMetric );
-		}
+	// The threshold of 50% is used because the limit for all beacons combined is 64 KiB, not just the data for one beacon.
+	if ( percentOfBudget < 50 ) {
+		log( message, urlMetric );
+	} else {
+		warn( message, urlMetric );
 	}
 
 	const url = new URL( restApiEndpoint );
@@ -807,12 +852,7 @@
 		);
 	}
 	url.searchParams.set( 'hmac', urlMetricHMAC );
-	navigator.sendBeacon(
-		url,
-		new Blob( [ jsonBody ], {
-			type: 'application/json',
-		} )
-	);
+	navigator.sendBeacon( url, payloadBlob );
 
 	// Clean up.
 	breadcrumbedElementsMap.clear();
Index: detection.php
===================================================================
--- detection.php	(revision 3257264)
+++ detection.php	(working copy)
@@ -121,7 +121,7 @@
 		'maxViewportAspectRatio' => od_get_maximum_viewport_aspect_ratio(),
 		'isDebug'                => WP_DEBUG,
 		'extensionModuleUrls'    => $extension_module_urls,
-		'restApiEndpoint'        => rest_url( OD_REST_API_NAMESPACE . OD_URL_METRICS_ROUTE ),
+		'restApiEndpoint'        => rest_url( OD_REST_URL_Metrics_Store_Endpoint::ROUTE_NAMESPACE . OD_REST_URL_Metrics_Store_Endpoint::ROUTE_BASE ),
 		'currentETag'            => $current_etag,
 		'currentUrl'             => $current_url,
 		'urlMetricSlug'          => $slug,
@@ -157,3 +157,65 @@
 		array( 'type' => 'module' )
 	);
 }
+
+/**
+ * Registers the REST API endpoint for storing URL Metrics.
+ *
+ * @since 1.0.0
+ * @access private
+ */
+function od_register_rest_url_metric_store_endpoint(): void {
+	$endpoint_controller = new OD_REST_URL_Metrics_Store_Endpoint();
+
+	register_rest_route(
+		$endpoint_controller::ROUTE_NAMESPACE,
+		$endpoint_controller::ROUTE_BASE,
+		$endpoint_controller->get_registration_args()
+	);
+}
+
+/**
+ * Triggers post update actions for page caches to invalidate their caches related to the supplied cache purge post ID.
+ *
+ * This is intended to flush any page cache for the URL after the new URL Metric was submitted so that the optimizations
+ * which depend on that URL Metric can start to take effect.
+ *
+ * @since 1.0.0
+ *
+ * @param positive-int $cache_purge_post_id Cache purge post ID.
+ */
+function od_trigger_post_update_actions( int $cache_purge_post_id ): void {
+
+	$post = get_post( $cache_purge_post_id );
+	if ( ! ( $post instanceof WP_Post ) ) {
+		return;
+	}
+
+	// Fire actions that page caching plugins listen to flush caches.
+
+	/*
+	* The clean_post_cache action is used to flush page caches by:
+	* - Pantheon Advanced Cache <https://github.com/pantheon-systems/pantheon-advanced-page-cache/blob/e3b5552b0cb9268d9b696cb200af56cc044920d9/pantheon-advanced-page-cache.php#L185>
+	* - WP Super Cache <https://github.com/Automattic/wp-super-cache/blob/73b428d2fce397fd874b3056ad3120c343bc1a0c/wp-cache-phase2.php#L1615>
+	* - Batcache <https://github.com/Automattic/batcache/blob/ed0e6b2d9bcbab3924c49a6c3247646fb87a0957/batcache.php#L18>
+	*/
+	/** This action is documented in wp-includes/post.php. */
+	do_action( 'clean_post_cache', $post->ID, $post );
+
+	/*
+	* The transition_post_status action is used to flush page caches by:
+	* - Jetpack Boost <https://github.com/Automattic/jetpack-boost-production/blob/4090a3f9414c2171cd52d8a397f00b0d1151475f/app/modules/optimizations/page-cache/pre-wordpress/Boost_Cache.php#L76>
+	* - WP Super Cache <https://github.com/Automattic/wp-super-cache/blob/73b428d2fce397fd874b3056ad3120c343bc1a0c/wp-cache-phase2.php#L1616>
+	* - LightSpeed Cache <https://github.com/litespeedtech/lscache_wp/blob/7c707469b3c88b4f45d9955593b92f9aeaed54c3/src/purge.cls.php#L68>
+	*/
+	/** This action is documented in wp-includes/post.php. */
+	do_action( 'transition_post_status', $post->post_status, $post->post_status, $post );
+
+	/*
+	* The clean_post_cache action is used to flush page caches by:
+	* - W3 Total Cache <https://github.com/BoldGrid/w3-total-cache/blob/ab08f104294c6a8dcb00f1c66aaacd0615c42850/Util_AttachToActions.php#L32>
+	* - WP Rocket <https://github.com/wp-media/wp-rocket/blob/e5bca6673a3669827f3998edebc0c785210fe561/inc/common/purge.php#L283>
+	*/
+	/** This action is documented in wp-includes/post.php. */
+	do_action( 'save_post', $post->ID, $post, /* $update */ true );
+}
Index: helper.php
===================================================================
--- helper.php	(revision 3257264)
+++ helper.php	(working copy)
@@ -22,22 +22,6 @@
 	/**
 	 * Fires when extensions to Optimization Detective can be loaded and initialized.
 	 *
-	 * This action is useful for loading extension code that depends on Optimization Detective to be running. The version
-	 * of the plugin is passed as the sole argument so that if the required version is not present, the callback can short circuit.
-	 *
-	 * Example:
-	 *
-	 *     add_action( 'od_init', function ( string $version ) {
-	 *         if ( version_compare( $version, '1.0', '<' ) ) {
-	 *             add_action( 'admin_notices', 'my_plugin_warn_optimization_plugin_outdated' );
-	 *             return;
-	 *         }
-	 *
-	 *         // Bootstrap the Optimization Detective extension.
-	 *         require_once __DIR__ . '/functions.php';
-	 *         // ...
-	 *     } );
-	 *
 	 * @since 0.7.0
 	 *
 	 * @param string $version Optimization Detective version.
Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -24,4 +24,6 @@
 add_filter( 'site_status_tests', 'od_add_rest_api_availability_test' );
 add_action( 'admin_init', 'od_maybe_run_rest_api_health_check' );
 add_action( 'after_plugin_row_meta', 'od_render_rest_api_health_check_admin_notice_in_plugin_row', 30 );
+add_action( 'rest_api_init', 'od_register_rest_url_metric_store_endpoint' );
+add_action( 'od_trigger_page_cache_invalidation', 'od_trigger_post_update_actions' );
 // @codeCoverageIgnoreEnd
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -5,7 +5,7 @@
  * Description: Provides a framework for leveraging real user metrics to detect optimizations for improving page performance.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 1.0.0-beta2
+ * Version: 1.0.0-beta3
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -71,7 +71,7 @@
 	}
 )(
 	'optimization_detective_pending_plugin',
-	'1.0.0-beta2',
+	'1.0.0-beta3',
 	static function ( string $version ): void {
 		if ( defined( 'OPTIMIZATION_DETECTIVE_VERSION' ) ) {
 			return;
@@ -99,6 +99,9 @@
 
 		require_once __DIR__ . '/helper.php';
 
+		// Deprecations.
+		require_once __DIR__ . '/deprecated.php';
+
 		// Core infrastructure classes.
 		require_once __DIR__ . '/class-od-data-validation-exception.php';
 		require_once __DIR__ . '/class-od-html-tag-processor.php';
@@ -114,7 +117,7 @@
 		require_once __DIR__ . '/storage/class-od-url-metrics-post-type.php';
 		require_once __DIR__ . '/storage/class-od-storage-lock.php';
 		require_once __DIR__ . '/storage/data.php';
-		require_once __DIR__ . '/storage/rest-api.php';
+		require_once __DIR__ . '/storage/class-od-rest-url-metrics-store-endpoint.php';
 		require_once __DIR__ . '/storage/class-od-url-metric-store-request-context.php';
 
 		// Detection logic.
@@ -121,6 +124,7 @@
 		require_once __DIR__ . '/detection.php';
 
 		// Optimization logic.
+		require_once __DIR__ . '/class-od-template-optimization-context.php';
 		require_once __DIR__ . '/class-od-link-collection.php';
 		require_once __DIR__ . '/class-od-tag-visitor-registry.php';
 		require_once __DIR__ . '/class-od-visited-tag-state.php';
Index: optimization.php
===================================================================
--- optimization.php	(revision 3257264)
+++ optimization.php	(working copy)
@@ -238,7 +238,7 @@
 	// If the initial tag is not an open HTML tag, then abort since the buffer is not a complete HTML document.
 	$processor = new OD_HTML_Tag_Processor( $buffer );
 	if ( ! (
-		$processor->next_tag() &&
+		$processor->next_tag( array( 'tag_closers' => 'visit' ) ) &&
 		! $processor->is_tag_closer() &&
 		'HTML' === $processor->get_tag()
 	) ) {
@@ -245,85 +245,22 @@
 		return $buffer;
 	}
 
-	$slug = od_get_url_metrics_slug( od_get_normalized_query_vars() );
-	$post = OD_URL_Metrics_Post_Type::get_post( $slug );
+	$query_vars = od_get_normalized_query_vars();
+	$slug       = od_get_url_metrics_slug( $query_vars );
+	$post       = OD_URL_Metrics_Post_Type::get_post( $slug );
 
+	/**
+	 * Post ID.
+	 *
+	 * @var positive-int|null $post_id
+	 */
+	$post_id = $post instanceof WP_Post ? $post->ID : null;
+
 	$tag_visitor_registry = new OD_Tag_Visitor_Registry();
 
 	/**
 	 * Fires to register tag visitors before walking over the document to perform optimizations.
 	 *
-	 * Once a page has finished rendering and the output buffer is processed, the page contents are loaded into
-	 * an HTML Tag Processor instance. It then iterates over each tag in the document, and at each open tag it will
-	 * invoke all registered tag visitors. A tag visitor is simply a callable (such as a regular function, closure,
-	 * or even a class with an `__invoke` method defined). The tag visitor callback is invoked by passing an instance
-	 * of the `OD_Tag_Visitor_Context` object which includes the following read-only properties:
-	 *
-	 * - `$processor` (`OD_HTML_Tag_Processor`): The processor with the cursor at the current open tag.
-	 * - `$url_metric_group_collection` (`OD_URL_Metric_Group_Collection`): The URL Metrics which may include information about the current tag to inform what optimizations the callback performs.
-	 * - `$link_collection` (`OD_Link_Collection`): Collection of links which will be added to the `HEAD` when the page is served. This allows you to add preload links and preconnect links as needed.
-	 * - `$url_metrics_id` (`positive-int|null`): The post ID for the `od_url_metrics` post from which the URL Metrics were loaded (if any). For advanced usage.
-	 *
-	 * Note that you are free to call `$processor->next_tag()` in the callback (such as to walk over any child elements)
-	 * since the tag processor's cursor will be reset to the tag after the callback finishes.
-	 *
-	 * When a tag visitor sees it is at a relevant open tag (e.g. by checking `$processor->get_tag()`), it can call the
-	 * `$context->track_tag()` method to indicate that the tag should be measured during detection. This will cause the
-	 * tag to be included among the `elements` in the stored URL Metrics. The element data includes properties such
-	 * as `intersectionRatio`, `intersectionRect`, and `boundingClientRect` (provided by an `IntersectionObserver`) as
-	 * well as whether the tag is the LCP element (`isLCP`) or LCP element candidate (`isLCPCandidate`). This method
-	 * should not be called if the current tag is not relevant for the tag visitor or if the tag visitor callback does
-	 * not need to query the provided `OD_URL_Metric_Group_Collection` instance to apply the desired optimizations. (In
-	 * addition to calling the `$context->track_tag()`, a callback may also return `true` to indicate the tag should be
-	 * tracked.)
-	 *
-	 * Here's an example tag visitor that depends on URL Metrics data:
-	 *
-	 *     $tag_visitor_registry->register(
-	 *         'lcp-img-fetchpriority-high',
-	 *         static function ( OD_Tag_Visitor_Context $context ): void {
-	 *             if ( $context->processor->get_tag() !== 'IMG' ) {
-	 *                 return; // Tag is not relevant for this tag visitor.
-	 *             }
-	 *
-	 *             // Mark the tag for measurement during detection so it is included among the elements stored in URL Metrics.
-	 *             $context->track_tag();
-	 *
-	 *             // Make sure fetchpriority=high is added to LCP IMG elements based on the captured URL Metrics.
-	 *             $common_lcp_element = $context->url_metric_group_collection->get_common_lcp_element();
-	 *             if (
-	 *                 null !== $common_lcp_element
-	 *                 &&
-	 *                 $common_lcp_element->get_xpath() === $context->processor->get_xpath()
-	 *             ) {
-	 *                 $context->processor->set_attribute( 'fetchpriority', 'high' );
-	 *             }
-	 *         }
-	 *     );
-	 *
-	 * Please note this implementation of setting `fetchpriority=high` on the LCP `IMG` element is simplified. Please
-	 * see the Image Prioritizer extension for a more robust implementation.
-	 *
-	 * Here's an example tag visitor that does not depend on any URL Metrics data:
-	 *
-	 *     $tag_visitor_registry->register(
-	 *         'img-decoding-async',
-	 *         static function ( OD_Tag_Visitor_Context $context ): bool {
-	 *             if ( $context->processor->get_tag() !== 'IMG' ) {
-	 *                 return; // Tag is not relevant for this tag visitor.
-	 *             }
-	 *
-	 *             // Set the decoding attribute if it is absent.
-	 *             if ( null === $context->processor->get_attribute( 'decoding' ) ) {
-	 *                 $context->processor->set_attribute( 'decoding', 'async' );
-	 *             }
-	 *         }
-	 *     );
-	 *
-	 * Refer to [Image Prioritizer](https://github.com/WordPress/performance/tree/trunk/plugins/image-prioritizer) and
-	 * [Embed Optimizer](https://github.com/WordPress/performance/tree/trunk/plugins/embed-optimizer) for additional
-	 * examples of how tag visitors are used.
-	 *
 	 * @since 0.3.0
 	 *
 	 * @param OD_Tag_Visitor_Registry $tag_visitor_registry Tag visitor registry.
@@ -330,8 +267,8 @@
 	 */
 	do_action( 'od_register_tag_visitors', $tag_visitor_registry );
 
-	$current_etag         = od_get_current_url_metrics_etag( $tag_visitor_registry, $wp_the_query, od_get_current_theme_template() );
-	$group_collection     = new OD_URL_Metric_Group_Collection(
+	$current_etag     = od_get_current_url_metrics_etag( $tag_visitor_registry, $wp_the_query, od_get_current_theme_template() );
+	$group_collection = new OD_URL_Metric_Group_Collection(
 		$post instanceof WP_Post ? OD_URL_Metrics_Post_Type::get_url_metrics_from_post( $post ) : array(),
 		$current_etag,
 		od_get_breakpoint_max_widths(),
@@ -338,7 +275,27 @@
 		od_get_url_metrics_breakpoint_sample_size(),
 		od_get_url_metric_freshness_ttl()
 	);
-	$link_collection      = new OD_Link_Collection();
+	$link_collection  = new OD_Link_Collection();
+
+	$template_optimization_context = new OD_Template_Optimization_Context(
+		$group_collection,
+		$link_collection,
+		$query_vars,
+		$slug,
+		$post_id
+	);
+
+	/**
+	 * Fires before Optimization Detective starts iterating over the document in the output buffer.
+	 *
+	 * This is before any of the registered tag visitors have been invoked.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param OD_Template_Optimization_Context $template_optimization_context Template optimization context.
+	 */
+	do_action( 'od_start_template_optimization', $template_optimization_context );
+
 	$visited_tag_state    = new OD_Visited_Tag_State();
 	$tag_visitor_context  = new OD_Tag_Visitor_Context(
 		$processor,
@@ -345,7 +302,7 @@
 		$group_collection,
 		$link_collection,
 		$visited_tag_state,
-		$post instanceof WP_Post && $post->ID > 0 ? $post->ID : null
+		$post_id
 	);
 	$current_tag_bookmark = 'optimization_detective_current_tag';
 	$visitors             = iterator_to_array( $tag_visitor_registry );
@@ -355,7 +312,12 @@
 
 	do {
 		// Never process anything inside NOSCRIPT since it will never show up in the DOM when scripting is enabled, and thus it can never be detected nor measured.
-		if ( in_array( 'NOSCRIPT', $processor->get_breadcrumbs(), true ) ) {
+		// Similarly, elements in the Admin Bar are not relevant for optimization, so this loop ensures that no tags in the Admin Bar are visited.
+		if (
+			in_array( 'NOSCRIPT', $processor->get_breadcrumbs(), true )
+			||
+			$processor->is_admin_bar()
+		) {
 			continue;
 		}
 
@@ -382,15 +344,31 @@
 		}
 
 		if ( $tracked_in_url_metrics && $needs_detection ) {
-			// TODO: Replace get_stored_xpath with get_xpath once the transitional period is over.
-			$xpath = $processor->get_stored_xpath();
-			$processor->set_meta_attribute( 'xpath', $xpath );
+			$processor->set_meta_attribute( 'xpath', $processor->get_xpath() );
 		}
 
 		$visited_tag_state->reset();
-	} while ( $processor->next_open_tag() );
+	} while ( $processor->next_tag( array( 'tag_closers' => 'skip' ) ) );
 
+	// Inject detection script.
+	// TODO: When optimizing above, if we find that there is a stored LCP element but it fails to match, it should perhaps set $needs_detection to true and send the request with an override nonce. However, this would require backtracking and adding the data-od-xpath attributes.
+	if ( $needs_detection ) {
+		$processor->append_body_html( od_get_detection_script( $slug, $group_collection ) );
+	}
+
+	/**
+	 * Fires after Optimization Detective has finished iterating over the document in the output buffer.
+	 *
+	 * This is after all the registered tag visitors have been invoked.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param OD_Template_Optimization_Context $template_optimization_context Template optimization context.
+	 */
+	do_action( 'od_finish_template_optimization', $template_optimization_context );
+
 	// Send any preload links in a Link response header and in a LINK tag injected at the end of the HEAD.
+	// Additional links may have been added at the od_finish_template_optimization action, so this must come after.
 	if ( count( $link_collection ) > 0 ) {
 		$response_header_links = $link_collection->get_response_header();
 		if ( ! is_null( $response_header_links ) && ! headers_sent() ) {
@@ -399,11 +377,5 @@
 		$processor->append_head_html( $link_collection->get_html() );
 	}
 
-	// Inject detection script.
-	// TODO: When optimizing above, if we find that there is a stored LCP element but it fails to match, it should perhaps set $needs_detection to true and send the request with an override nonce. However, this would require backtracking and adding the data-od-xpath attributes.
-	if ( $needs_detection ) {
-		$processor->append_body_html( od_get_detection_script( $slug, $group_collection ) );
-	}
-
 	return $processor->get_updated_html();
 }
Index: readme.txt
===================================================================
--- readme.txt	(revision 3257264)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   1.0.0-beta2
+Stable tag:   1.0.0-beta3
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, optimization, rum
@@ -13,11 +13,11 @@
 
 This plugin captures real user metrics about what elements are displayed on the page across a variety of device form factors (e.g. desktop, tablet, and phone) in order to apply loading optimizations which are not possible with WordPress’s current server-side heuristics.
 
-This plugin is a dependency which does not provide end-user functionality on its own. For that, please install the dependent plugin [Image Prioritizer](https://wordpress.org/plugins/image-prioritizer/) or [Embed Optimizer](https://wordpress.org/plugins/embed-optimizer/) (among [others](https://github.com/WordPress/performance/labels/%5BPlugin%5D%20Optimization%20Detective) to come from the WordPress Core Performance team). There are currently **no settings** and no user interface for this plugin since it is designed to work without any configuration.
+This plugin is a framework dependency which does not provide optimization functionality on its own. For that, please install the [Image Prioritizer](https://wordpress.org/plugins/image-prioritizer/) and [Embed Optimizer](https://wordpress.org/plugins/embed-optimizer/) dependent plugins (among [others](https://github.com/WordPress/performance/labels/%5BPlugin%5D%20Optimization%20Detective) to come from the WordPress Core Performance team). There are currently **no settings** and no user interface for this plugin since it is designed to work without any configuration.
 
-Your site must have the **REST API accessible** to frontend visitors since this is how metrics are collected about how a page should be optimized.
+Your site must currently have the **REST API accessible** to unauthenticated frontend visitors since this is how real user metrics are collected about pages on your site; nevertheless, [exploration](https://github.com/WordPress/performance/issues/1311) is underway for providing alternative mechanisms for collecting the metrics. Also, please note that no metrics are currently collected from Safari since it does not yet support the [Largest Contentful Paint](https://developer.mozilla.org/en-US/docs/Web/API/LargestContentfulPaint) metric, but support for this [Core Web Vitals](https://web.dev/explore/learn-core-web-vitals) metric is coming this year [via Interop 2025](https://webkit.org/blog/16458/announcing-interop-2025/#core-web-vitals).
 
-Please refer to the [full plugin documentation](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/README.md) for a [technical introduction](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/introduction.md), [filter/action hooks](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/hooks.md), and [extensions](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/extensions.md) that show use cases and examples.
+Please refer to the [full plugin documentation](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/README.md) for a [technical introduction](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/introduction.md), [filter/action hooks](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/hooks.md), and [extensions](https://github.com/WordPress/performance/blob/trunk/plugins/optimization-detective/docs/extensions.md) that show use cases and examples. You can also watch the talk from WordCamp Asia 2025: [Boosting Performance with Optimization Detective](https://weston.ruter.net/2025/02/21/boosting-performance-with-optimization-detective/).
 
 == Installation ==
 
@@ -55,6 +55,23 @@
 
 == Changelog ==
 
+= 1.0.0-beta3 =
+
+**Enhancements**
+
+* Fire actions before and after Optimization Detective processes a document. ([1919](https://github.com/WordPress/performance/pull/1919))
+* Update `OD_HTML_Tag_Processor::next_tag()` to allow `$query` arg and prepare to skip visiting tag closers by default. ([1872](https://github.com/WordPress/performance/pull/1872))
+* Expose the logging functions to client-side extensions and automatically account for the value of `isDebug`. ([1895](https://github.com/WordPress/performance/pull/1895))
+* Update URL Metric storage REST API endpoint to return status code `423 Locked` instead of `403 Forbidden`. ([1863](https://github.com/WordPress/performance/pull/1863))
+* De-duplicate logic between REST API and URL Metrics post type. ([1867](https://github.com/WordPress/performance/pull/1867))
+
+**Bug Fixes**
+
+* Fix URL encoding in Link HTTP response header. ([1907](https://github.com/WordPress/performance/pull/1907))
+* Fix triggering post update actions after storing a URL Metric and refactor REST API endpoint logic into class. ([1865](https://github.com/WordPress/performance/pull/1865))
+* Fix unpredictable LCP element being identified in a URL Metric Group. ([1903](https://github.com/WordPress/performance/pull/1903))
+* Handle missing Web Crypto API in non-HTTPS contexts when generating the already-submitted `sessionStorage` key. ([1911](https://github.com/WordPress/performance/pull/1911))
+
 = 1.0.0-beta2 =
 
 **Enhancements**
Index: site-health.php
===================================================================
--- site-health.php	(revision 3257264)
+++ site-health.php	(working copy)
@@ -91,7 +91,7 @@
 		sprintf(
 			/* translators: %s is the REST API endpoint */
 			__( 'To collect URL Metrics from visitors the REST API must be available to unauthenticated users. Specifically, visitors must be able to perform a <code>POST</code> request to the <code>%s</code> endpoint.', 'optimization-detective' ),
-			'/' . OD_REST_API_NAMESPACE . OD_URL_METRICS_ROUTE
+			'/' . OD_REST_URL_Metrics_Store_Endpoint::ROUTE_NAMESPACE . OD_REST_URL_Metrics_Store_Endpoint::ROUTE_BASE
 		),
 		array( 'code' => array() )
 	) . '</p>';
@@ -192,7 +192,7 @@
 	if ( false !== $response ) {
 		return $response;
 	}
-	$rest_url = get_rest_url( null, OD_REST_API_NAMESPACE . OD_URL_METRICS_ROUTE );
+	$rest_url = get_rest_url( null, OD_REST_URL_Metrics_Store_Endpoint::ROUTE_NAMESPACE . OD_REST_URL_Metrics_Store_Endpoint::ROUTE_BASE );
 	$response = wp_remote_post(
 		$rest_url,
 		array(
Index: storage/class-od-url-metrics-post-type.php
===================================================================
--- storage/class-od-url-metrics-post-type.php	(revision 3257264)
+++ storage/class-od-url-metrics-post-type.php	(working copy)
@@ -200,22 +200,36 @@
 	}
 
 	/**
-	 * Stores URL Metric by merging it with the other URL Metrics which share the same normalized query vars.
+	 * Inserts or updates the od_url_metrics post with a provided collection of URL Metrics.
 	 *
-	 * @since 0.1.0
-	 * @todo There is duplicate logic here with od_handle_rest_request().
+	 * This method updates an existing URL Metrics post or creates a new one if it doesn't exist.
 	 *
-	 * @param non-empty-string $slug Slug (hash of normalized query vars).
-	 * @param OD_URL_Metric    $new_url_metric New URL Metric.
-	 * @return positive-int|WP_Error Post ID or WP_Error otherwise.
+	 * @since 1.0.0
+	 *
+	 * @param non-empty-string               $slug Slug (hash of normalized query vars).
+	 * @param OD_URL_Metric_Group_Collection $url_metric_group_collection URL Metric group collection containing the metrics to be stored.
+	 * @return positive-int|WP_Error Post ID on success, or WP_Error on failure.
 	 */
-	public static function store_url_metric( string $slug, OD_URL_Metric $new_url_metric ) {
-		$post_data = array(
+	public static function update_post( string $slug, OD_URL_Metric_Group_Collection $url_metric_group_collection ) {
+		$url_metrics = $url_metric_group_collection->get_flattened_url_metrics();
+		if ( 0 === count( $url_metrics ) ) {
+			return new WP_Error( 'no_url_metrics', __( 'No URL Metrics in the group collection.', 'optimization-detective' ) );
+		}
+
+		// Sort URL Metrics in descending order by timestamp.
+		usort(
+			$url_metrics,
+			static function ( OD_URL_Metric $a, OD_URL_Metric $b ): int {
+				return $b->get_timestamp() <=> $a->get_timestamp();
+			}
+		);
+		$latest_url_metric = $url_metrics[0];
+		$post_data         = array(
 			// The URL is supplied as the post title in order to aid with debugging. Note that an od-url-metrics post stores
 			// multiple URL Metric instances, each of which also contains the URL for which the metric was captured. The URL
 			// appearing in the post title is therefore the most recent URL seen for the URL Metrics which have the same
 			// normalized query vars among them.
-			'post_title' => $new_url_metric->get_url(),
+			'post_title' => $latest_url_metric->get_url(),
 		);
 
 		$post = self::get_post( $slug );
@@ -222,31 +236,12 @@
 		if ( $post instanceof WP_Post ) {
 			$post_data['ID']        = $post->ID;
 			$post_data['post_name'] = $post->post_name;
-			$url_metrics            = self::get_url_metrics_from_post( $post );
 		} else {
 			$post_data['post_name'] = $slug;
-			$url_metrics            = array();
 		}
 
-		$etag = $new_url_metric->get_etag();
-
-		$group_collection = new OD_URL_Metric_Group_Collection(
-			$url_metrics,
-			$etag,
-			od_get_breakpoint_max_widths(),
-			od_get_url_metrics_breakpoint_sample_size(),
-			od_get_url_metric_freshness_ttl()
-		);
-
-		try {
-			$group = $group_collection->get_group_for_viewport_width( $new_url_metric->get_viewport_width() );
-			$group->add_url_metric( $new_url_metric );
-		} catch ( InvalidArgumentException $e ) {
-			return new WP_Error( 'invalid_url_metric', $e->getMessage() );
-		}
-
 		$post_data['post_content'] = wp_json_encode(
-			$group_collection->get_flattened_url_metrics(),
+			$url_metric_group_collection->get_flattened_url_metrics(),
 			JSON_UNESCAPED_SLASHES // No need for escaping slashes since this JSON is not embedded in HTML.
 		);
 		if ( ! is_string( $post_data['post_content'] ) ) {
Index: storage/data.php
===================================================================
--- storage/data.php	(revision 3257264)
+++ storage/data.php	(working copy)
@@ -37,7 +37,7 @@
 
 	if ( $freshness_ttl < 0 ) {
 		_doing_it_wrong(
-			__FUNCTION__,
+			esc_html( "Filter: 'od_url_metric_freshness_ttl'" ),
 			esc_html(
 				sprintf(
 					/* translators: %s is the TTL freshness */
@@ -382,15 +382,13 @@
  * @return positive-int[] Breakpoint max widths, sorted in ascending order.
  */
 function od_get_breakpoint_max_widths(): array {
-	$function_name = __FUNCTION__;
-
 	$breakpoint_max_widths = array_map(
-		static function ( $original_breakpoint ) use ( $function_name ): int {
+		static function ( $original_breakpoint ): int {
 			$breakpoint = $original_breakpoint;
 			if ( $breakpoint <= 0 ) {
 				$breakpoint = 1;
 				_doing_it_wrong(
-					esc_html( $function_name ),
+					esc_html( "Filter: 'od_breakpoint_max_widths'" ),
 					esc_html(
 						sprintf(
 							/* translators: %s is the actual breakpoint max width */
@@ -447,7 +445,7 @@
 
 	if ( $sample_size <= 0 ) {
 		_doing_it_wrong(
-			__FUNCTION__,
+			esc_html( "Filter: 'od_url_metrics_breakpoint_sample_size'" ),
 			esc_html(
 				sprintf(
 					/* translators: %s is the sample size */
Index: types.ts
===================================================================
--- types.ts	(revision 3257264)
+++ types.ts	(working copy)
@@ -1,13 +1,13 @@
 // h/t https://stackoverflow.com/a/59801602/93579
 type ExcludeProps< T > = { [ k: string ]: any } & { [ K in keyof T ]?: never };
 
-import { onTTFB, onFCP, onLCP, onINP, onCLS } from 'web-vitals';
+import { onCLS, onFCP, onINP, onLCP, onTTFB } from 'web-vitals';
 import {
-	onTTFB as onTTFBWithAttribution,
+	onCLS as onCLSWithAttribution,
 	onFCP as onFCPWithAttribution,
+	onINP as onINPWithAttribution,
 	onLCP as onLCPWithAttribution,
-	onINP as onINPWithAttribution,
-	onCLS as onCLSWithAttribution,
+	onTTFB as onTTFBWithAttribution,
 } from 'web-vitals/attribution';
 
 export interface ElementData {
@@ -49,8 +49,21 @@
 export type OnINPWithAttributionFunction = typeof onINPWithAttribution;
 export type OnCLSWithAttributionFunction = typeof onCLSWithAttribution;
 
+export type LogFunction = ( ...message: any[] ) => void;
+
+export interface Logger {
+	log: LogFunction;
+	info: LogFunction;
+	warn: LogFunction;
+	error: LogFunction;
+}
+
 export type InitializeArgs = {
 	readonly isDebug: boolean;
+	readonly log: LogFunction;
+	readonly info: LogFunction;
+	readonly warn: LogFunction;
+	readonly error: LogFunction;
 	readonly onTTFB: OnTTFBFunction | OnTTFBWithAttributionFunction;
 	readonly onFCP: OnFCPFunction | OnFCPWithAttributionFunction;
 	readonly onLCP: OnLCPFunction | OnLCPWithAttributionFunction;
@@ -69,11 +82,16 @@
 		properties: ExtendedElementData
 	) => void;
 	readonly isDebug: boolean;
+	readonly log: LogFunction;
+	readonly info: LogFunction;
+	readonly warn: LogFunction;
+	readonly error: LogFunction;
 };
 
 export type FinalizeCallback = ( args: FinalizeArgs ) => Promise< void >;
 
 export interface Extension {
-	initialize?: InitializeCallback;
-	finalize?: FinalizeCallback;
+	readonly name?: string;
+	readonly initialize?: InitializeCallback;
+	readonly finalize?: FinalizeCallback;
 }

Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -12,48 +12,18 @@
 }
 // @codeCoverageIgnoreEnd
 
-/**
- * Prints the speculation rules.
- *
- * For browsers that do not support speculation rules yet, the `script[type="speculationrules"]` tag will be ignored.
- *
- * @since 1.0.0
- */
-function plsr_print_speculation_rules(): void {
-	// Skip speculative loading for logged-in users.
-	if ( is_user_logged_in() ) {
-		return;
-	}
+// Conditionally use either the WordPress Core API, or load the plugin's API implementation otherwise.
+if ( function_exists( 'wp_get_speculation_rules_configuration' ) ) {
+	require_once __DIR__ . '/wp-core-api.php';
 
-	// Skip speculative loading for sites without pretty permalinks, unless explicitly enabled.
-	if ( ! (bool) get_option( 'permalink_structure' ) ) {
-		/**
-		 * Filters whether speculative loading should be enabled even though the site does not use pretty permalinks.
-		 *
-		 * Since query parameters are commonly used by plugins for dynamic behavior that can change state, ideally any
-		 * such URLs are excluded from speculative loading. If the site does not use pretty permalinks though, they are
-		 * impossible to recognize. Therefore speculative loading is disabled by default for those sites.
-		 *
-		 * For site owners of sites without pretty permalinks that are certain their site is not using such a pattern,
-		 * this filter can be used to still enable speculative loading at their own risk.
-		 *
-		 * @since 1.4.0
-		 *
-		 * @param bool $enabled Whether speculative loading is enabled even without pretty permalinks.
-		 */
-		$enabled = (bool) apply_filters( 'plsr_enabled_without_pretty_permalinks', false );
+	add_filter( 'wp_speculation_rules_configuration', 'plsr_filter_speculation_rules_configuration' );
+	add_filter( 'wp_speculation_rules_href_exclude_paths', 'plsr_filter_speculation_rules_exclude_paths', 10, 2 );
+} else {
+	require_once __DIR__ . '/class-plsr-url-pattern-prefixer.php';
+	require_once __DIR__ . '/plugin-api.php';
 
-		if ( ! $enabled ) {
-			return;
-		}
-	}
-
-	wp_print_inline_script_tag(
-		(string) wp_json_encode( plsr_get_speculation_rules() ),
-		array( 'type' => 'speculationrules' )
-	);
+	add_action( 'wp_footer', 'plsr_print_speculation_rules' );
 }
-add_action( 'wp_footer', 'plsr_print_speculation_rules' );
 
 /**
  * Displays the HTML generator meta tag for the Speculative Loading plugin.
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -5,7 +5,7 @@
  * Description: Enables browsers to speculatively prerender or prefetch pages to achieve near-instant loads based on user interaction.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 1.4.0
+ * Version: 1.5.0
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -19,7 +19,6 @@
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
-// @codeCoverageIgnoreEnd
 
 (
 	/**
@@ -66,7 +65,7 @@
 	}
 )(
 	'plsr_pending_plugin_info',
-	'1.4.0',
+	'1.5.0',
 	static function ( string $version ): void {
 
 		// Define the constant.
@@ -77,9 +76,8 @@
 		define( 'SPECULATION_RULES_VERSION', $version );
 		define( 'SPECULATION_RULES_MAIN_FILE', plugin_basename( __FILE__ ) );
 
-		require_once __DIR__ . '/class-plsr-url-pattern-prefixer.php';
-		require_once __DIR__ . '/helper.php';
 		require_once __DIR__ . '/hooks.php';
 		require_once __DIR__ . '/settings.php';
 	}
 );
+// @codeCoverageIgnoreEnd
Index: readme.txt
===================================================================
--- readme.txt	(revision 3257264)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   1.4.0
+Stable tag:   1.5.0
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, javascript, speculation rules, prerender, prefetch
@@ -119,6 +119,12 @@
 
 == Changelog ==
 
+= 1.5.0 =
+
+**Enhancements**
+
+* Add support for Speculative Loading WP Core API, loading the plugin's own API implementation conditionally. ([1883](https://github.com/WordPress/performance/pull/1883))
+
 = 1.4.0 =
 
 **Enhancements**
Index: settings.php
===================================================================
--- settings.php	(revision 3257264)
+++ settings.php	(working copy)
@@ -224,7 +224,8 @@
 			$choices = plsr_get_eagerness_labels();
 			break;
 		default:
-			return; // Invalid (and this case should never occur).
+			// Invalid (and this case should never occur).
+			return; // @codeCoverageIgnore
 	}
 
 	$value = $option[ $args['field'] ];
Index: uninstall.php
===================================================================
--- uninstall.php	(revision 3257264)
+++ uninstall.php	(working copy)
@@ -8,7 +8,7 @@
 
 // If uninstall.php is not called by WordPress, bail.
 if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
-	exit;
+	exit;// @codeCoverageIgnore
 }
 
 // For a multisite, delete the option for all sites (however limited to 100 sites to avoid memory limit or timeout problems in large scale networks).

Index: helper.php
===================================================================
--- helper.php	(revision 3257264)
+++ helper.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Gets configuration for Web Worker Offloading.
@@ -23,11 +25,11 @@
 	$config = array(
 		// The source code in the build directory is compiled from <https://github.com/BuilderIO/partytown/tree/main/src/lib>.
 		// See webpack config in the WordPress/performance repo: <https://github.com/WordPress/performance/blob/282a068f3eb2575d37aeb9034e894e7140fcddca/webpack.config.js#L84-L130>.
-		'lib' => wp_parse_url( plugin_dir_url( __FILE__ ), PHP_URL_PATH ) . 'build/',
+		'lib' => wp_parse_url( plugins_url( 'build/', __FILE__ ), PHP_URL_PATH ),
 	);
 
 	if ( WP_DEBUG && SCRIPT_DEBUG ) {
-		$config['debug'] = true;
+		$config['debug'] = true;// @codeCoverageIgnore
 	}
 
 	/**
@@ -70,3 +72,129 @@
 	 */
 	return (array) apply_filters( 'plwwo_configuration', $config );
 }
+
+/**
+ * Registers defaults scripts for Web Worker Offloading.
+ *
+ * @since 0.1.0
+ * @access private
+ *
+ * @param WP_Scripts $scripts WP_Scripts instance.
+ */
+function plwwo_register_default_scripts( WP_Scripts $scripts ): void {
+	// The source code for partytown.js is built from <https://github.com/BuilderIO/partytown/blob/b292a14047a0c12ca05ba97df1833935d42fdb66/src/lib/main/snippet.ts>.
+	// See webpack config in the WordPress/performance repo: <https://github.com/WordPress/performance/blob/282a068f3eb2575d37aeb9034e894e7140fcddca/webpack.config.js#L84-L130>.
+	if ( defined( 'SCRIPT_DEBUG' ) && SCRIPT_DEBUG ) {
+		$partytown_js_path = '/build/debug/partytown.js';// @codeCoverageIgnore
+	} else {
+		$partytown_js_path = '/build/partytown.js';
+	}
+
+	$partytown_js = file_get_contents( __DIR__ . $partytown_js_path ); // phpcs:ignore WordPress.WP.AlternativeFunctions.file_get_contents_file_get_contents -- It's a local filesystem path not a remote request.
+	if ( false === $partytown_js ) {
+		return;// @codeCoverageIgnore
+	}
+
+	$scripts->add(
+		'web-worker-offloading',
+		'',
+		array(),
+		WEB_WORKER_OFFLOADING_VERSION,
+		array( 'in_footer' => false )
+	);
+
+	$scripts->add_inline_script(
+		'web-worker-offloading',
+		sprintf(
+			'window.partytown = {...(window.partytown || {}), ...%s};',
+			wp_json_encode( plwwo_get_configuration() )
+		),
+		'before'
+	);
+
+	$scripts->add_inline_script( 'web-worker-offloading', $partytown_js );
+}
+
+/**
+ * Prepends web-worker-offloading to the list of scripts to print if one of the queued scripts is offloaded to a worker.
+ *
+ * @since 0.1.0
+ * @access private
+ *
+ * @param string[]|mixed $script_handles An array of enqueued script dependency handles.
+ * @return string[] Script handles.
+ */
+function plwwo_filter_print_scripts_array( $script_handles ): array {
+	$scripts = wp_scripts();
+	foreach ( (array) $script_handles as $handle ) {
+		if ( true === (bool) $scripts->get_data( $handle, 'worker' ) ) {
+			$scripts->set_group( 'web-worker-offloading', false, 0 ); // Try to print in the head.
+			array_unshift( $script_handles, 'web-worker-offloading' );
+			break;
+		}
+	}
+	return $script_handles;
+}
+
+/**
+ * Updates script type for handles having `web-worker-offloading` as dependency.
+ *
+ * @since 0.1.0
+ * @access private
+ *
+ * @param string|mixed $tag    Script tag.
+ * @param string       $handle Script handle.
+ * @return string|mixed Script tag with type="text/partytown" for eligible scripts.
+ */
+function plwwo_update_script_type( $tag, string $handle ) {
+	if (
+		is_string( $tag )
+		&&
+		(bool) wp_scripts()->get_data( $handle, 'worker' )
+	) {
+		$html_processor = new WP_HTML_Tag_Processor( $tag );
+		while ( $html_processor->next_tag( array( 'tag_name' => 'SCRIPT' ) ) ) {
+			if ( $html_processor->get_attribute( 'id' ) === "{$handle}-js" ) {
+				$html_processor->set_attribute( 'type', 'text/partytown' );
+				$tag = $html_processor->get_updated_html();
+				break;
+			}
+		}
+	}
+	return $tag;
+}
+
+/**
+ * Filters inline script attributes to offload to a worker if the script has been opted-in.
+ *
+ * @since 0.1.0
+ * @access private
+ *
+ * @param array<string, mixed>|mixed $attributes Attributes.
+ * @return array<string, mixed> Attributes.
+ */
+function plwwo_filter_inline_script_attributes( $attributes ): array {
+	$attributes = (array) $attributes;
+	if (
+		isset( $attributes['id'] )
+		&&
+		1 === preg_match( '/^(?P<handle>.+)-js-(?:before|after)$/', $attributes['id'], $matches )
+		&&
+		(bool) wp_scripts()->get_data( $matches['handle'], 'worker' )
+	) {
+		$attributes['type'] = 'text/partytown';
+	}
+	return $attributes;
+}
+
+/**
+ * Displays the HTML generator meta tag for the Web Worker Offloading plugin.
+ *
+ * See {@see 'wp_head'}.
+ *
+ * @since 0.1.1
+ */
+function plwwo_render_generator_meta_tag(): void {
+	// Use the plugin slug as it is immutable.
+	echo '<meta name="generator" content="web-worker-offloading ' . esc_attr( WEB_WORKER_OFFLOADING_VERSION ) . '">' . "\n";
+}
Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -6,137 +6,13 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
-
-/**
- * Registers defaults scripts for Web Worker Offloading.
- *
- * @since 0.1.0
- * @access private
- *
- * @param WP_Scripts $scripts WP_Scripts instance.
- */
-function plwwo_register_default_scripts( WP_Scripts $scripts ): void {
-	// The source code for partytown.js is built from <https://github.com/BuilderIO/partytown/blob/b292a14047a0c12ca05ba97df1833935d42fdb66/src/lib/main/snippet.ts>.
-	// See webpack config in the WordPress/performance repo: <https://github.com/WordPress/performance/blob/282a068f3eb2575d37aeb9034e894e7140fcddca/webpack.config.js#L84-L130>.
-	if ( defined( 'SCRIPT_DEBUG' ) && SCRIPT_DEBUG ) {
-		$partytown_js_path = '/build/debug/partytown.js';
-	} else {
-		$partytown_js_path = '/build/partytown.js';
-	}
-
-	$partytown_js = file_get_contents( __DIR__ . $partytown_js_path ); // phpcs:ignore WordPress.WP.AlternativeFunctions.file_get_contents_file_get_contents -- It's a local filesystem path not a remote request.
-	if ( false === $partytown_js ) {
-		return;
-	}
-
-	$scripts->add(
-		'web-worker-offloading',
-		'',
-		array(),
-		WEB_WORKER_OFFLOADING_VERSION,
-		array( 'in_footer' => false )
-	);
-
-	$scripts->add_inline_script(
-		'web-worker-offloading',
-		sprintf(
-			'window.partytown = {...(window.partytown || {}), ...%s};',
-			wp_json_encode( plwwo_get_configuration() )
-		),
-		'before'
-	);
-
-	$scripts->add_inline_script( 'web-worker-offloading', $partytown_js );
-}
 add_action( 'wp_default_scripts', 'plwwo_register_default_scripts' );
-
-/**
- * Prepends web-worker-offloading to the list of scripts to print if one of the queued scripts is offloaded to a worker.
- *
- * @since 0.1.0
- * @access private
- *
- * @param string[]|mixed $script_handles An array of enqueued script dependency handles.
- * @return string[] Script handles.
- */
-function plwwo_filter_print_scripts_array( $script_handles ): array {
-	$scripts = wp_scripts();
-	foreach ( (array) $script_handles as $handle ) {
-		if ( true === (bool) $scripts->get_data( $handle, 'worker' ) ) {
-			$scripts->set_group( 'web-worker-offloading', false, 0 ); // Try to print in the head.
-			array_unshift( $script_handles, 'web-worker-offloading' );
-			break;
-		}
-	}
-	return $script_handles;
-}
 add_filter( 'print_scripts_array', 'plwwo_filter_print_scripts_array', PHP_INT_MAX );
-
-/**
- * Updates script type for handles having `web-worker-offloading` as dependency.
- *
- * @since 0.1.0
- * @access private
- *
- * @param string|mixed $tag    Script tag.
- * @param string       $handle Script handle.
- * @return string|mixed Script tag with type="text/partytown" for eligible scripts.
- */
-function plwwo_update_script_type( $tag, string $handle ) {
-	if (
-		is_string( $tag )
-		&&
-		(bool) wp_scripts()->get_data( $handle, 'worker' )
-	) {
-		$html_processor = new WP_HTML_Tag_Processor( $tag );
-		while ( $html_processor->next_tag( array( 'tag_name' => 'SCRIPT' ) ) ) {
-			if ( $html_processor->get_attribute( 'id' ) === "{$handle}-js" ) {
-				$html_processor->set_attribute( 'type', 'text/partytown' );
-				$tag = $html_processor->get_updated_html();
-				break;
-			}
-		}
-	}
-	return $tag;
-}
 add_filter( 'script_loader_tag', 'plwwo_update_script_type', 10, 2 );
-
-/**
- * Filters inline script attributes to offload to a worker if the script has been opted-in.
- *
- * @since 0.1.0
- * @access private
- *
- * @param array<string, mixed>|mixed $attributes Attributes.
- * @return array<string, mixed> Attributes.
- */
-function plwwo_filter_inline_script_attributes( $attributes ): array {
-	$attributes = (array) $attributes;
-	if (
-		isset( $attributes['id'] )
-		&&
-		1 === preg_match( '/^(?P<handle>.+)-js-(?:before|after)$/', $attributes['id'], $matches )
-		&&
-		(bool) wp_scripts()->get_data( $matches['handle'], 'worker' )
-	) {
-		$attributes['type'] = 'text/partytown';
-	}
-	return $attributes;
-}
 add_filter( 'wp_inline_script_attributes', 'plwwo_filter_inline_script_attributes' );
-
-/**
- * Displays the HTML generator meta tag for the Web Worker Offloading plugin.
- *
- * See {@see 'wp_head'}.
- *
- * @since 0.1.1
- */
-function plwwo_render_generator_meta_tag(): void {
-	// Use the plugin slug as it is immutable.
-	echo '<meta name="generator" content="web-worker-offloading ' . esc_attr( WEB_WORKER_OFFLOADING_VERSION ) . '">' . "\n";
-}
 add_action( 'wp_head', 'plwwo_render_generator_meta_tag' );
+// @codeCoverageIgnoreEnd
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -15,9 +15,9 @@
  * @package web-worker-offloading
  */
 
-// Exit if accessed directly.
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
-	exit;
+	exit; // Exit if accessed directly.
 }
 
 // Define the constant.
@@ -48,3 +48,4 @@
 require_once __DIR__ . '/helper.php';
 require_once __DIR__ . '/hooks.php';
 require_once __DIR__ . '/third-party.php';
+// @codeCoverageIgnoreEnd
Index: third-party/google-site-kit.php
===================================================================
--- third-party/google-site-kit.php	(revision 3257264)
+++ third-party/google-site-kit.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Configures WWO for Site Kit and Google Analytics.
@@ -41,7 +43,7 @@
 
 	return $configuration;
 }
-add_filter( 'plwwo_configuration', 'plwwo_google_site_kit_configure' );
+add_filter( 'plwwo_configuration', 'plwwo_google_site_kit_configure' ); // @codeCoverageIgnore
 
 plwwo_mark_scripts_for_offloading(
 	array(
@@ -68,4 +70,4 @@
 	return $attributes;
 }
 
-add_filter( 'wp_inline_script_attributes', 'plwwo_google_site_kit_filter_inline_script_attributes' );
+add_filter( 'wp_inline_script_attributes', 'plwwo_google_site_kit_filter_inline_script_attributes' ); // @codeCoverageIgnore
Index: third-party/seo-by-rank-math.php
===================================================================
--- third-party/seo-by-rank-math.php	(revision 3257264)
+++ third-party/seo-by-rank-math.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Configures WWO for Rank Math SEO and Google Analytics.
@@ -30,7 +32,7 @@
 	$configuration['forward'][] = 'gtag';
 	return $configuration;
 }
-add_filter( 'plwwo_configuration', 'plwwo_rank_math_configure' );
+add_filter( 'plwwo_configuration', 'plwwo_rank_math_configure' ); // @codeCoverageIgnore
 
 /*
  * Note: The following integration is not targeting the \RankMath\Analytics\GTag::enqueue_gtag_js() code which is only
@@ -58,7 +60,7 @@
 	return $attributes;
 }
 
-add_filter( 'wp_script_attributes', 'plwwo_rank_math_filter_script_attributes' );
+add_filter( 'wp_script_attributes', 'plwwo_rank_math_filter_script_attributes' ); // @codeCoverageIgnore
 
 /**
  * Filters inline script attributes to offload Rank Math's GTag script tag to Partytown.
@@ -78,4 +80,4 @@
 	return $attributes;
 }
 
-add_filter( 'wp_inline_script_attributes', 'plwwo_rank_math_filter_inline_script_attributes' );
+add_filter( 'wp_inline_script_attributes', 'plwwo_rank_math_filter_inline_script_attributes' ); // @codeCoverageIgnore
Index: third-party/woocommerce.php
===================================================================
--- third-party/woocommerce.php	(revision 3257264)
+++ third-party/woocommerce.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Configures WWO for WooCommerce and Google Analytics.
@@ -31,7 +33,7 @@
 
 	return $configuration;
 }
-add_filter( 'plwwo_configuration', 'plwwo_woocommerce_configure' );
+add_filter( 'plwwo_configuration', 'plwwo_woocommerce_configure' ); // @codeCoverageIgnore
 
 plwwo_mark_scripts_for_offloading(
 	// Note: 'woocommerce-google-analytics-integration' is intentionally not included because for some reason events like add_to_cart don't get tracked.
Index: third-party.php
===================================================================
--- third-party.php	(revision 3257264)
+++ third-party.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Adds scripts to be offloaded to a worker.

Index: deprecated.php
===================================================================
--- deprecated.php	(revision 3257264)
+++ deprecated.php	(working copy)
@@ -30,12 +30,17 @@
 	$metadata = wp_get_attachment_metadata( $attachment_id );
 
 	// Return full image size sources.
-	if ( 'full' === $size && ! empty( $metadata['sources'] ) ) {
+	if (
+		'full' === $size &&
+		isset( $metadata['sources'] ) &&
+		is_array( $metadata['sources'] ) &&
+		count( $metadata['sources'] ) > 0
+	) {
 		return $metadata['sources'];
 	}
 
 	// Return the resized image sources.
-	if ( ! empty( $metadata['sizes'][ $size ]['sources'] ) ) {
+	if ( isset( $metadata['sizes'][ $size ]['sources'] ) && is_array( $metadata['sizes'][ $size ]['sources'] ) ) {
 		return $metadata['sizes'][ $size ]['sources'];
 	}
 
Index: helper.php
===================================================================
--- helper.php	(revision 3257264)
+++ helper.php	(working copy)
@@ -66,7 +66,7 @@
 	// Ensure that all mime types have correct transforms. If a mime type has invalid transforms array,
 	// then fallback to the original mime type to make sure that the correct subsizes are created.
 	foreach ( $transforms as $mime_type => $transform_types ) {
-		if ( ! is_array( $transform_types ) || empty( $transform_types ) ) {
+		if ( ! is_array( $transform_types ) || 0 === count( $transform_types ) ) {
 			$transforms[ $mime_type ] = array( $mime_type );
 		}
 	}
@@ -82,11 +82,11 @@
  * @since 1.0.0
  * @access private
  *
- * @param int                                          $attachment_id         The ID of the attachment from where this image would be created.
- * @param string                                       $image_size            The size name that would be used to create the image source, out of the registered subsizes.
- * @param array{ width: int, height: int, crop: bool } $size_data             An array with the dimensions of the image: height, width and crop.
- * @param string                                       $mime                  The target mime in which the image should be created.
- * @param string|null                                  $destination_file_name The path where the file would be stored, including the extension. If null, `generate_filename` is used to create the destination file name.
+ * @param int                                                               $attachment_id         The ID of the attachment from where this image would be created.
+ * @param string                                                            $image_size            The size name that would be used to create the image source, out of the registered subsizes.
+ * @param array{ width: int, height: int, crop: bool|array{string, string}} $size_data             An array with the dimensions of the image: height, width and crop.
+ * @param string                                                            $mime                  The target mime in which the image should be created.
+ * @param string|null                                                       $destination_file_name The path where the file would be stored, including the extension. If null, `generate_filename` is used to create the destination file name.
  *
  * @return array{ file: string, filesize: int }|WP_Error An array with the file and filesize if the image was created correctly, otherwise a WP_Error.
  */
@@ -109,7 +109,7 @@
 	 * @param array{
 	 *            width: int,
 	 *            height: int,
-	 *            crop: bool
+	 *            crop: bool|array{string, string}
 	 *        }               $size_data     An array with the dimensions of the image.
 	 * @param string          $mime          The target mime in which the image should be created.
 	 */
@@ -156,7 +156,7 @@
 
 	$height = isset( $size_data['height'] ) ? (int) $size_data['height'] : 0;
 	$width  = isset( $size_data['width'] ) ? (int) $size_data['width'] : 0;
-	$crop   = isset( $size_data['crop'] ) && $size_data['crop'];
+	$crop   = isset( $size_data['crop'] ) ? $size_data['crop'] : false;
 	if ( $width <= 0 && $height <= 0 ) {
 		return new WP_Error( 'image_wrong_dimensions', __( 'At least one of the dimensions must be a positive number.', 'webp-uploads' ) );
 	}
@@ -163,7 +163,7 @@
 
 	$image_meta = wp_get_attachment_metadata( $attachment_id );
 	// If stored EXIF data exists, rotate the source image before creating sub-sizes.
-	if ( ! empty( $image_meta['image_meta'] ) ) {
+	if ( isset( $image_meta['image_meta'] ) && is_array( $image_meta['image_meta'] ) && count( $image_meta['image_meta'] ) > 0 ) {
 		$editor->maybe_exif_rotate();
 	}
 
@@ -185,7 +185,7 @@
 		return $image;
 	}
 
-	if ( empty( $image['file'] ) ) {
+	if ( ! isset( $image['file'] ) || ! is_string( $image['file'] ) || '' === $image['file'] ) {
 		return new WP_Error( 'image_file_not_present', __( 'The file key is not present on the image data', 'webp-uploads' ) );
 	}
 
@@ -241,7 +241,7 @@
 	}
 
 	if ( isset( $sizes[ $size ]['crop'] ) ) {
-		$size_data['crop'] = (bool) $sizes[ $size ]['crop'];
+		$size_data['crop'] = $sizes[ $size ]['crop'];
 	}
 
 	return webp_uploads_generate_additional_image_source( $attachment_id, $size, $size_data, $mime );
Index: hooks.php
===================================================================
--- hooks.php	(revision 3257264)
+++ hooks.php	(working copy)
@@ -77,7 +77,7 @@
 		$metadata['sources'] = array();
 	}
 
-	if ( empty( $metadata['sources'][ $mime_type ] ) ) {
+	if ( ! isset( $metadata['sources'][ $mime_type ]['file'] ) ) {
 		$metadata['sources'][ $mime_type ] = array(
 			'file'     => wp_basename( $file ),
 			'filesize' => wp_filesize( $file ),
@@ -99,12 +99,12 @@
 	// Create the sources for the full sized image.
 	foreach ( $valid_mime_transforms[ $mime_type ] as $targeted_mime ) {
 		// If this property exists no need to create the image again.
-		if ( ! empty( $metadata['sources'][ $targeted_mime ] ) ) {
+		if ( isset( $metadata['sources'][ $targeted_mime ]['file'] ) ) {
 			continue;
 		}
 
 		// The targeted mime is not allowed in the current installation.
-		if ( empty( $allowed_mimes[ $targeted_mime ] ) ) {
+		if ( ! isset( $allowed_mimes[ $targeted_mime ] ) ) {
 			continue;
 		}
 
@@ -150,7 +150,7 @@
 			$original_image = wp_get_original_image_path( $attachment_id );
 
 			// If WordPress already modified the original itself, keep the original and discard WordPress's generated version.
-			if ( ! empty( $metadata['original_image'] ) ) {
+			if ( isset( $metadata['original_image'] ) && is_string( $metadata['original_image'] ) && '' !== $metadata['original_image'] ) {
 				$uploadpath    = wp_get_upload_dir();
 				$attached_file = get_attached_file( $attachment_id );
 				if ( false !== $attached_file ) {
@@ -171,7 +171,11 @@
 	}
 
 	// Make sure we have some sizes to work with, otherwise avoid any work.
-	if ( empty( $metadata['sizes'] ) || ! is_array( $metadata['sizes'] ) ) {
+	if (
+		! isset( $metadata['sizes'] ) ||
+		! is_array( $metadata['sizes'] ) ||
+		0 === count( $metadata['sizes'] )
+	) {
 		return $metadata;
 	}
 
@@ -179,7 +183,11 @@
 
 	foreach ( $metadata['sizes'] as $size_name => $properties ) {
 		// Do nothing if this image size is not an array or is not allowed to have additional mime types.
-		if ( ! is_array( $properties ) || empty( $sizes_with_mime_type_support[ $size_name ] ) ) {
+		if (
+			! is_array( $properties ) ||
+			! isset( $sizes_with_mime_type_support[ $size_name ] ) ||
+			false === $sizes_with_mime_type_support[ $size_name ]
+		) {
 			continue;
 		}
 
@@ -192,16 +200,16 @@
 		}
 
 		// The mime type can't be determined.
-		if ( empty( $current_mime ) ) {
+		if ( ! is_string( $current_mime ) || '' === $current_mime ) {
 			continue;
 		}
 
 		// Ensure a `sources` property exists on the existing size.
-		if ( empty( $properties['sources'] ) || ! is_array( $properties['sources'] ) ) {
+		if ( ! isset( $properties['sources'] ) || ! is_array( $properties['sources'] ) ) {
 			$properties['sources'] = array();
 		}
 
-		if ( empty( $properties['sources'][ $current_mime ] ) && isset( $properties['file'] ) ) {
+		if ( ! isset( $properties['sources'][ $current_mime ]['file'] ) && isset( $properties['file'] ) ) {
 			$properties['sources'][ $current_mime ] = array(
 				'file'     => $properties['file'],
 				'filesize' => 0,
@@ -217,7 +225,7 @@
 
 		foreach ( $valid_mime_transforms[ $mime_type ] as $mime ) {
 			// If this property exists no need to create the image again.
-			if ( ! empty( $properties['sources'][ $mime ] ) ) {
+			if ( isset( $properties['sources'][ $mime ]['file'] ) ) {
 				continue;
 			}
 
@@ -275,7 +283,7 @@
 	}
 
 	// Only setup the trace array if we no longer have more sizes.
-	if ( ! empty( $missing_sizes ) ) {
+	if ( count( $missing_sizes ) > 0 ) {
 		return $missing_sizes;
 	}
 
@@ -362,7 +370,7 @@
 function webp_uploads_remove_sources_files( int $attachment_id ): void {
 	$file = get_attached_file( $attachment_id );
 
-	if ( empty( $file ) ) {
+	if ( false === (bool) $file ) {
 		return;
 	}
 
@@ -371,7 +379,11 @@
 	$sizes = ! isset( $metadata['sizes'] ) || ! is_array( $metadata['sizes'] ) ? array() : $metadata['sizes'];
 
 	$upload_path = wp_get_upload_dir();
-	if ( empty( $upload_path['basedir'] ) ) {
+	if (
+		! isset( $upload_path['basedir'] ) ||
+		! is_string( $upload_path['basedir'] ) ||
+		'' === $upload_path['basedir']
+	) {
 		return;
 	}
 
@@ -383,7 +395,7 @@
 			continue;
 		}
 
-		$original_size_mime = empty( $size['mime-type'] ) ? '' : $size['mime-type'];
+		$original_size_mime = isset( $size['mime-type'] ) && is_string( $size['mime-type'] ) ? $size['mime-type'] : '';
 
 		foreach ( $size['sources'] as $mime => $properties ) {
 			/**
@@ -397,7 +409,11 @@
 				continue;
 			}
 
-			if ( ! is_array( $properties ) || empty( $properties['file'] ) ) {
+			if (
+				! isset( $properties['file'] ) ||
+				! is_string( $properties['file'] ) ||
+				'' === $properties['file']
+			) {
 				continue;
 			}
 
@@ -429,7 +445,11 @@
 			continue;
 		}
 
-		if ( ! is_array( $properties ) || empty( $properties['file'] ) ) {
+		if (
+			! isset( $properties['file'] ) ||
+			! is_string( $properties['file'] ) ||
+			'' === $properties['file']
+		) {
 			continue;
 		}
 
@@ -453,7 +473,7 @@
 			continue;
 		}
 
-		$original_backup_size_mime = empty( $backup_size['mime-type'] ) ? '' : $backup_size['mime-type'];
+		$original_backup_size_mime = isset( $backup_size['mime-type'] ) && is_string( $backup_size['mime-type'] ) ? $backup_size['mime-type'] : '';
 
 		foreach ( $backup_size['sources'] as $backup_mime => $backup_properties ) {
 			/**
@@ -467,12 +487,16 @@
 				continue;
 			}
 
-			if ( ! is_array( $backup_properties ) || empty( $backup_properties['file'] ) ) {
+			if (
+				! isset( $backup_properties['file'] ) ||
+				! is_string( $backup_properties['file'] ) ||
+				'' === $backup_properties['file']
+			) {
 				continue;
 			}
 
 			$backup_intermediate_file = str_replace( $basename, $backup_properties['file'], $file );
-			if ( empty( $backup_intermediate_file ) ) {
+			if ( '' === $backup_intermediate_file ) {
 				continue;
 			}
 
@@ -492,12 +516,16 @@
 	foreach ( $backup_sources as $backup_mimes ) {
 
 		foreach ( $backup_mimes as $backup_mime_properties ) {
-			if ( ! is_array( $backup_mime_properties ) || empty( $backup_mime_properties['file'] ) ) {
+			if (
+				! isset( $backup_mime_properties['file'] ) ||
+				! is_string( $backup_mime_properties['file'] ) ||
+				'' === $backup_mime_properties['file']
+			) {
 				continue;
 			}
 
 			$full_size = str_replace( $basename, $backup_mime_properties['file'], $file );
-			if ( empty( $full_size ) ) {
+			if ( '' === $full_size ) {
 				continue;
 			}
 
@@ -547,7 +575,7 @@
 	$image    = $original_image;
 	$metadata = wp_get_attachment_metadata( $attachment_id );
 
-	if ( empty( $metadata['file'] ) ) {
+	if ( ! isset( $metadata['file'] ) || ! is_string( $metadata['file'] ) || '' === $metadata['file'] ) {
 		return $image;
 	}
 
@@ -596,7 +624,7 @@
 		// Replace sub sizes for the image if present.
 		foreach ( $metadata['sizes'] as $size => $size_data ) {
 
-			if ( empty( $size_data['file'] ) ) {
+			if ( ! isset( $size_data['file'] ) || ! is_string( $size_data['file'] ) || '' === $size_data['file'] ) {
 				continue;
 			}
 
@@ -671,7 +699,7 @@
 	);
 
 	foreach ( $additional_sizes as $size => $size_details ) {
-		$allowed_sizes[ $size ] = ! empty( $size_details['provide_additional_mime_types'] );
+		$allowed_sizes[ $size ] = isset( $size_details['provide_additional_mime_types'] ) && true === (bool) $size_details['provide_additional_mime_types'];
 	}
 
 	/**
Index: image-edit.php
===================================================================
--- image-edit.php	(revision 3257264)
+++ image-edit.php	(working copy)
@@ -86,7 +86,11 @@
 		}
 
 		foreach ( $metadata['sizes'] as $size_name => $size_details ) {
-			if ( empty( $subsized_images[ $targeted_mime ][ $size_name ]['file'] ) ) {
+			if (
+				! isset( $subsized_images[ $targeted_mime ][ $size_name ]['file'] ) ||
+				! is_string( $subsized_images[ $targeted_mime ][ $size_name ]['file'] ) ||
+				'' === $subsized_images[ $targeted_mime ][ $size_name ]['file']
+			) {
 				continue;
 			}
 
@@ -124,7 +128,7 @@
 	}
 
 	$transforms = webp_uploads_get_upload_image_mime_transforms();
-	if ( empty( $transforms[ $mime_type ] ) ) {
+	if ( ! isset( $transforms[ $mime_type ] ) || ! is_array( $transforms[ $mime_type ] ) || 0 === count( $transforms[ $mime_type ] ) ) {
 		return null;
 	}
 
@@ -144,7 +148,7 @@
 			}
 			$callback_executed = true;
 			// No sizes to be created.
-			if ( empty( $metadata['sizes'] ) ) {
+			if ( ! isset( $metadata['sizes'] ) || ! is_array( $metadata['sizes'] ) || 0 === count( $metadata['sizes'] ) ) {
 				return $metadata;
 			}
 
@@ -163,7 +167,7 @@
 					}
 
 					if (
-						isset( $metadata['sizes'][ $size_name ] ) && ! empty( $metadata['sizes'][ $size_name ] ) &&
+						isset( $metadata['sizes'][ $size_name ]['file'] ) &&
 						$metadata['sizes'][ $size_name ]['file'] !== $old_metadata['sizes'][ $size_name ]['file']
 					) {
 						$resize_sizes[ $size_name ] = $metadata['sizes'][ $size_name ];
@@ -405,7 +409,7 @@
  * @param array<string, array{ file: string, filesize: int }> $sources       An array with the full sources to be stored on the next available key.
  */
 function webp_uploads_backup_full_image_sources( int $attachment_id, array $sources ): void {
-	if ( empty( $sources ) ) {
+	if ( 0 === count( $sources ) ) {
 		return;
 	}
 
@@ -435,7 +439,7 @@
 	$backup_sizes = get_post_meta( $attachment_id, '_wp_attachment_backup_sizes', true );
 	$backup_sizes = is_array( $backup_sizes ) ? $backup_sizes : array();
 
-	if ( empty( $backup_sizes ) ) {
+	if ( 0 === count( $backup_sizes ) ) {
 		return null;
 	}
 
Index: load.php
===================================================================
--- load.php	(revision 3257264)
+++ load.php	(working copy)
@@ -5,7 +5,7 @@
  * Description: Converts images to more modern formats such as WebP or AVIF during upload.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 2.5.0
+ * Version: 2.5.1
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -26,7 +26,7 @@
 	return;
 }
 
-define( 'WEBP_UPLOADS_VERSION', '2.5.0' );
+define( 'WEBP_UPLOADS_VERSION', '2.5.1' );
 define( 'WEBP_UPLOADS_MAIN_FILE', plugin_basename( __FILE__ ) );
 
 require_once __DIR__ . '/helper.php';
Index: picture-element.php
===================================================================
--- picture-element.php	(revision 3257264)
+++ picture-element.php	(working copy)
@@ -41,14 +41,33 @@
 		array_unshift( $image_sizes, $image_meta );
 	}
 
+	// Extract sizes using regex to parse image tag, then use to retrieve tag.
+	$width     = 0;
+	$height    = 0;
+	$processor = new WP_HTML_Tag_Processor( $image );
+	if ( $processor->next_tag( array( 'tag_name' => 'IMG' ) ) ) {
+		$width  = (int) $processor->get_attribute( 'width' );
+		$height = (int) $processor->get_attribute( 'height' );
+	}
+	$size_to_use = ( $width > 0 && $height > 0 ) ? array( $width, $height ) : 'full';
+
+	$image_src = wp_get_attachment_image_src( $attachment_id, $size_to_use );
+	if ( false === $image_src ) {
+		return $image;
+	}
+	list( $src, $width, $height ) = $image_src;
+	$size_array                   = array( absint( $width ), absint( $height ) );
+
 	// Collect all the sub size image mime types.
 	$mime_type_data = array();
 	foreach ( $image_sizes as $size ) {
 		if ( isset( $size['sources'] ) && isset( $size['width'] ) && isset( $size['height'] ) ) {
 			foreach ( $size['sources'] as $mime_type => $data ) {
-				$mime_type_data[ $mime_type ]                         = $mime_type_data[ $mime_type ] ?? array();
-				$mime_type_data[ $mime_type ]['w'][ $size['width'] ]  = $data;
-				$mime_type_data[ $mime_type ]['h'][ $size['height'] ] = $data;
+				if ( wp_image_matches_ratio( $size_array[0], $size_array[1], $size['width'], $size['height'] ) ) {
+					$mime_type_data[ $mime_type ]                         = $mime_type_data[ $mime_type ] ?? array();
+					$mime_type_data[ $mime_type ]['w'][ $size['width'] ]  = $data;
+					$mime_type_data[ $mime_type ]['h'][ $size['height'] ] = $data;
+				}
 			}
 		}
 	}
@@ -97,23 +116,6 @@
 	// Add each mime type to the picture's sources.
 	$picture_sources = '';
 
-	// Extract sizes using regex to parse image tag, then use to retrieve tag.
-	$width     = 0;
-	$height    = 0;
-	$processor = new WP_HTML_Tag_Processor( $image );
-	if ( $processor->next_tag( array( 'tag_name' => 'IMG' ) ) ) {
-		$width  = (int) $processor->get_attribute( 'width' );
-		$height = (int) $processor->get_attribute( 'height' );
-	}
-	$size_to_use = ( $width > 0 && $height > 0 ) ? array( $width, $height ) : 'full';
-
-	$image_src = wp_get_attachment_image_src( $attachment_id, $size_to_use );
-	if ( false === $image_src ) {
-		return $image;
-	}
-	list( $src, $width, $height ) = $image_src;
-	$size_array                   = array( absint( $width ), absint( $height ) );
-
 	// Gets the srcset and sizes from the IMG tag.
 	$sizes  = $processor->get_attribute( 'sizes' );
 	$srcset = $processor->get_attribute( 'srcset' );
Index: readme.txt
===================================================================
--- readme.txt	(revision 3257264)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   2.5.0
+Stable tag:   2.5.1
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, images, webp, avif, modern image formats
@@ -60,6 +60,13 @@
 
 == Changelog ==
 
+= 2.5.1 =
+
+**Bug Fixes**
+
+* Fix Modern Image Format not cropping image if crop is an array. ([1887](https://github.com/WordPress/performance/pull/1887))
+* Fix incorrect image size selection in `PICTURE` element. ([1885](https://github.com/WordPress/performance/pull/1885))
+
 = 2.5.0 =
 
 **Enhancements**
Index: rest-api.php
===================================================================
--- rest-api.php	(revision 3257264)
+++ rest-api.php	(working copy)
@@ -30,7 +30,7 @@
 
 	foreach ( $data['media_details']['sizes'] as $size => &$details ) {
 
-		if ( empty( $details['sources'] ) || ! is_array( $details['sources'] ) ) {
+		if ( ! isset( $details['sources'] ) || ! is_array( $details['sources'] ) ) {
 			continue;
 		}
 
@@ -41,7 +41,13 @@
 	}
 
 	$full_src = wp_get_attachment_image_src( $post->ID, 'full' );
-	if ( ! empty( $full_src ) && ! empty( $data['media_details']['sources'] ) && ! empty( $data['media_details']['sizes']['full'] ) ) {
+	if (
+		isset( $full_src[0] ) &&
+		isset( $data['media_details']['sources'] ) &&
+		is_array( $data['media_details']['sources'] ) &&
+		isset( $data['media_details']['sizes']['full'] ) &&
+		is_array( $data['media_details']['sizes']['full'] )
+	) {
 		$full_url_basename = wp_basename( $full_src[0] );
 		foreach ( $data['media_details']['sources'] as $mime => &$mime_details ) {
 			$mime_details['source_url'] = str_replace( $full_url_basename, $mime_details['file'], $full_src[0] );