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README.md

css-inline

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css-inline is a high-performance library for inlining CSS into HTML 'style' attributes.

This library is designed for scenarios such as preparing HTML emails or embedding HTML into third-party web pages.

For instance, the library transforms HTML like this:

<html>
  <head>
    <style>h1 { color:blue; }</style>
  </head>
  <body>
    <h1>Big Text</h1>
  </body>
</html>

into:

<html>
  <head></head>
  <body>
    <h1 style="color:blue;">Big Text</h1>
  </body>
</html>
  • Uses reliable components from Mozilla's Servo project
  • Inlines CSS from style and link tags
  • Removes style and link tags
  • Resolves external stylesheets (including local files)
  • Optionally caches external stylesheets
  • Works on Linux, Windows, and macOS
  • Supports HTML5 & CSS3

Playground

If you'd like to try css-inline, you can check the WebAssembly-powered playground to see the results instantly.

Install

The C bindings are distributed as a header (css_inline.h) along with a dynamic library (libcss_inline.so).

To download them, go to Releases and get the latest archive with the [C] tag.

Usage

#include "css_inline.h"
#include <stdio.h>

#define OUTPUT_SIZE 1024

int main(void) {
  CssInlinerOptions options = css_inliner_default_options();
  const char input[] =
    "<html>"
      "<head>"
        "<style>h1 {color : red}</style>"
      "</head>"
      "<body>"
        "<h1>Test</h1>"
      "</body>"
    "</ html>";
  char output[OUTPUT_SIZE];
  if (css_inline_to(&options, input, output, sizeof(output)) == CSS_RESULT_OK) {
    printf("Inlined CSS: %s\n", output);
  }

  // Alternatively, because CSS_RESULT_OK is equal to 0, you can do
  CssResult res = css_inline_to(&options, input, output, sizeof(output));
  if (!res) {
    printf("An error occurred while inlining the CSS, see the result enum type: %d", res);
  }

  return 0;
}

The inline function, css_inline_to(), doesn't allocate, so you must provide an array big enough to fit the result. If the size is not sufficient, the enum CSS_RESULT_IO_ERROR will be returned.

Note that css-inline automatically adds missing html and body tags, so the output is a valid HTML document.

Alternatively, you can inline CSS into an HTML fragment. Structural tags (<html>, <head>, <body>) are stripped from the output; only their contents are preserved. Use css_inline_to if you need to keep the full document structure:

#include "css_inline.h"
#include <stdio.h>

#define OUTPUT_SIZE 1024

int main(void) {
  CssInlinerOptions options = css_inliner_default_options();
  const char fragment[] =
    "<main>"
      "<h1>Hello</h1>"
      "<section>"
      "<p>who am i</p>"
      "</section>"
    "</main>";

  const char css[] =
    "p {"
      "color: red;"
    "}"
    "h1 {"
      "color: blue;"
    "}";
  char output[OUTPUT_SIZE];
  if (css_inline_fragment_to(&options, fragment, css, output, sizeof(output)) == CSS_RESULT_OK) {
    printf("Inlined CSS: %s\n", output);
    // HTML becomes this:
    // <main>
    // <h1 style="color: blue;">Hello</h1>
    // <section>
    // <p style="color: red;">who am i</p>
    // </section>
    // </main>
  }
  return 0;
}

Configuration

You can change the inline behavior by modifying the CssInlinerOptions struct parameter that will be passed to css_inline_to():

#include "css_inline.h"
#include <stdbool.h>

int main(void) {
  CssInlinerOptions options = css_inliner_default_options();
  options.load_remote_stylesheets = true;
  char input[] = "...";
  char output[256];
  if (!css_inline_to(&options, input, output, sizeof(output))) {
    // Deal with the error
  }
  return 0;
}

Possible configurations:

  • inline_style_tags. Specifies whether to inline CSS from "style" tags. Default: true
  • keep_style_tags. Specifies whether to keep "style" tags after inlining. Default: false
  • keep_link_tags. Specifies whether to keep "link" tags after inlining. Default: false
  • keep_at_rules. Specifies whether to keep "at-rules" (starting with @) after inlining. Default: false
  • minify_css. Specifies whether to remove trailing semicolons and spaces between properties and values.
  • base_url. The base URL used to resolve relative URLs. If you'd like to load stylesheets from your filesystem, use the file:// scheme. Default: NULL
  • load_remote_stylesheets. Specifies whether remote stylesheets should be loaded. Default: true
  • cache. Specifies caching options for external stylesheets. Default: NULL
  • extra_css. Extra CSS to be inlined. Default: NULL
  • preallocate_node_capacity. Advanced. Preallocates capacity for HTML nodes during parsing. This can improve performance when you have an estimate of the number of nodes in your HTML document. Default: 32
  • remove_inlined_selectors. Specifies whether to remove selectors that were successfully inlined from <style> blocks. Default: false
  • apply_width_attributes. Specifies whether to add width HTML attributes from CSS width properties on supported elements (table, td, th, img). Default: false
  • apply_height_attributes. Specifies whether to add height HTML attributes from CSS height properties on supported elements (table, td, th, img). Default: false

You can also skip CSS inlining for an HTML tag by adding the data-css-inline="ignore" attribute to it:

<html>
  <head>
    <style>h1 { color:blue; }</style>
  </head>
  <body>
    <!-- The tag below won't receive additional styles -->
    <h1 data-css-inline="ignore">Big Text</h1>
  </body>
</html>

The data-css-inline="ignore" attribute also allows you to skip link and style tags:

<head>
  <!-- Styles below are ignored -->
  <style data-css-inline="ignore">h1 { color:blue; }</style>
</head>
<body>
  <h1>Big Text</h1>
</body>

Alternatively, you may keep style from being removed by using the data-css-inline="keep" attribute. This is useful if you want to keep @media queries for responsive emails in separate style tags. Such tags will be kept in the resulting HTML even if the keep_style_tags option is set to false.

<head>
  <!-- Styles below are not removed -->
  <style data-css-inline="keep">h1 { color:blue; }</style>
</head>
<body>
  <h1>Big Text</h1>
</body>

Another possibility is to set keep_at_rules option to true. At-rules cannot be inlined into HTML therefore they get removed by default. This is useful if you want to keep at-rules, e.g. @media queries for responsive emails in separate style tags but inline any styles which can be inlined. Such tags will be kept in the resulting HTML even if the keep_style_tags option is explicitly set to false.

<head>
  <!-- With keep_at_rules=true "color:blue" will get inlined into <h1> but @media will be kept in <style> -->
  <style>h1 { color: blue; } @media (max-width: 600px) { h1 { font-size: 18px; } }</style>
</head>
<body>
  <h1>Big Text</h1>
</body>

If you set the the minify_css option to true, the inlined styles will be minified by removing trailing semicolons and spaces between properties and values.

<head>
  <!-- With minify_css=true, the <h1> will have `style="color:blue;font-weight:bold"` -->
  <style>h1 { color: blue; font-weight: bold; }</style>
</head>
<body>
  <h1>Big Text</h1>
</body>

You can also cache external stylesheets to avoid excessive network requests:

int main(void) {
  // Configure cache
  StylesheetCache cache = css_inliner_stylesheet_cache(8);
  CssInlinerOptions options = css_inliner_default_options();
  options.cache = &cache;
  // ... Inline CSS
  return 0;
}

Caching is disabled by default.

License

This project is licensed under the terms of the MIT license.