Skip to content

ruby

Directory actions

More options

Directory actions

More options

Latest commit

 

History

History

ruby

README.md

css_inline

build status ruby gems codecov.io gitter

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
  • Can process multiple documents in parallel
  • Works on Linux, Windows, and macOS
  • Supports HTML5 & CSS3
  • Tested on Ruby 3.2, 3.3, and 3.4.

Playground

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

Installation

Add this line to your application's Gemfile:

gem 'css_inline'

Usage

To inline CSS in an HTML document:

require 'css_inline'

html = "<html><head><style>h1 { color:blue; }</style></head><body><h1>Big Text</h1></body></html>"
inlined = CSSInline.inline(html)

puts inlined
# Outputs: "<html><head></head><body><h1 style=\"color:blue;\">Big Text</h1></body></html>"

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 CSSInline.inline if you need to keep the full document structure:

require 'css_inline'

fragment = """
<main>
<h1>Hello</h1>
<section>
<p>who am i</p>
</section>
</main>
"""

css = """
p {
    color: red;
}

h1 {
    color: blue;
}
"""

inlined = CSSInline.inline_fragment(fragment, css)

puts inlined
# HTML becomes this:
# <main>
# <h1 style="color: blue;">Hello</h1>
# <section>
# <p style="color: red;">who am i</p>
# </section>
# </main>

When there is a need to inline multiple HTML documents simultaneously, css_inline offers inline_many and inline_many_fragments functions. This feature allows for concurrent processing of several inputs, significantly improving performance when dealing with a large number of documents.

require 'css_inline'

inlined = CSSInline.inline_many(["...", "..."])

Under the hood, inline_many, spawns threads at the Rust layer to handle the parallel processing of inputs. This results in faster execution times compared to employing parallel processing techniques at the Ruby level.

Note: To fully benefit from inline_many, you should run your application on a multicore machine.

Configuration

For customization options use the CSSInliner class:

require 'css_inline'

inliner = CSSInline::CSSInliner.new(keep_style_tags: true)
inliner.inline("...")
  • 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. Default: false
  • 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: nil
  • load_remote_stylesheets. Specifies whether remote stylesheets should be loaded. Default: true
  • cache. Specifies caching options for external stylesheets (for example, StylesheetCache(size: 5)). Default: nil
  • extra_css. Extra CSS to be inlined. Default: nil
  • 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:

<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>

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>

If you'd like to load stylesheets from your filesystem, use the file:// scheme:

require 'css_inline'

# styles/email is relative to the current directory
inliner = CSSInline::CSSInliner.new(base_url: "file://styles/email/")
inliner.inline("...")

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

require 'css_inline'

inliner = CSSInline::CSSInliner.new(
    cache: CSSInline::StylesheetCache.new(size: 5)
)
inliner.inline("...")

Caching is disabled by default.

Performance

This library uses components from Mozilla's Servo project for CSS parsing and matching. Performance benchmarks show 50-100x faster execution than roadie and premailer.

The table below shows benchmark results comparing css_inline, roadie, and premailer on typical HTML documents:

Size css_inline 0.19.0 roadie 5.2.1 premailer 1.21.0
Basic usage 230 B 6.07 µs 173.50 µs (28.60x) 345.30 µs (56.91x)
Realistic email 1 8.58 KB 91.23 µs 713.40 µs (7.82x) 6.80 ms (74.53x)
Realistic email 2 4.3 KB 57.56 µs 1.99 ms (34.56x) ERROR
GitHub Page 1.81 MB 21.61 ms 8.20 s (379.45x) 2.40 s (111.06x)

Please refer to the test/bench.rb file to review the benchmark code. The results displayed above were measured using stable rustc 1.91 on Ruby 3.4.7.

Further reading

If you want to know how this library was created & how it works internally, you could take a look at these articles:

License

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