css_inline

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, preserving the original 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
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

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'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 0.15.0`	`roadie 5.2.1`	`premailer 1.21.0`
Basic usage	230 B	6.11 µs	175.21 µs (28.75)	350.10 µs (57.24x)
Realistic email 1	8.58 KB	101.82 µs	721.18 µs (6.96x)	7.99 ms (78.49x)
Realistic email 2	4.3 KB	77.22 µs	2.02 ms (28.48x)	ERROR
GitHub Page	1.81 MB	172.41 ms	8.84 s (52.21x)	2.77 s (15.97x)

Please refer to the test/bench.rb file to review the benchmark code. The results displayed above were measured using stable rustc 1.87 on Ruby 3.3.5.

License

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