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GitHub - Stranger6667/css-inline: High-performance library for inlining CSS into HTML 'style' attributes

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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 crate 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>
  • Extremely fast with a minimal memory footprint
  • 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
  • Bindings for Python, Ruby, JavaScript, Java, C, PHP, and a WebAssembly module to run in browsers.
  • Elixir bindings maintained by Knock
  • Command Line Interface

Playground

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

Installation

To include it in your project, add the following line to the dependencies section in your project's Cargo.toml file:

[dependencies]
css-inline = "0.20"

The Minimum Supported Rust Version is 1.85.

Usage

const HTML: &str = r#"<html>
<head>
    <style>h1 { color:blue; }</style>
</head>
<body>
    <h1>Big Text</h1>
</body>
</html>"#;

fn main() -> css_inline::Result<()> {
    let inlined = css_inline::inline(HTML)?;
    // Do something with inlined HTML, e.g. send an email
    Ok(())
}

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 inline if you need to keep the full document structure:

const FRAGMENT: &str = r#"<main>
<h1>Hello</h1>
<section>
<p>who am i</p>
</section>
</main>"#;

const CSS: &str = r#"
p {
    color: red;
}

h1 {
    color: blue;
}
"#;

fn main() -> css_inline::Result<()> {
    let inlined = css_inline::inline_fragment(FRAGMENT, CSS)?;
    Ok(())
}

Configuration

css-inline can be configured by using CSSInliner::options() that implements the Builder pattern:

const HTML: &str = "...";

fn main() -> css_inline::Result<()> {
    let inliner = css_inline::CSSInliner::options()
        .load_remote_stylesheets(false)
        .build();
    let inlined = inliner.inline(HTML)?;
    // Do something with inlined HTML, e.g. send an email
    Ok(())
}
  • 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: None
  • load_remote_stylesheets. Specifies whether remote stylesheets should be loaded. Default: true
  • cache. Specifies cache for external stylesheets. Default: None
  • extra_css. Extra CSS to be inlined. Default: None
  • 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:

const HTML: &str = "...";

fn main() -> css_inline::Result<()> {
    let base_url = css_inline::Url::parse("file://styles/email/").expect("Invalid URL");
    let inliner = css_inline::CSSInliner::options()
        .base_url(Some(base_url))
        .build();
    let inlined = inliner.inline(HTML);
    // Do something with inlined HTML, e.g. send an email
    Ok(())
}

For resolving remote stylesheets it is possible to implement a custom resolver:

#[derive(Debug, Default)]
pub struct CustomStylesheetResolver;

impl css_inline::StylesheetResolver for CustomStylesheetResolver {
    fn retrieve(&self, location: &str) -> css_inline::Result<String> {
        Err(self.unsupported("External stylesheets are not supported"))
    }
}

fn main() -> css_inline::Result<()> {
    let inliner = css_inline::CSSInliner::options()
        .resolver(std::sync::Arc::new(CustomStylesheetResolver))
        .build();
    Ok(())
}

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

use std::num::NonZeroUsize;

#[cfg(feature = "stylesheet-cache")]
fn main() -> css_inline::Result<()> {
    let inliner = css_inline::CSSInliner::options()
        .cache(
            // This is an LRU cache
            css_inline::StylesheetCache::new(
                NonZeroUsize::new(5).expect("Invalid cache size")
            )
        )
        .build();
    Ok(())
}

// This block is here for testing purposes
#[cfg(not(feature = "stylesheet-cache"))]
fn main() -> css_inline::Result<()> {
    Ok(())
}

Caching is disabled by default.

Performance

css-inline typically inlines HTML emails within hundreds of microseconds, though results may vary with input complexity.

Benchmarks for css-inline==0.20.0:

  • Basic: 4.09 µs, 230 bytes
  • Realistic-1: 78.94 µs, 8.58 KB
  • Realistic-2: 48.56 µs, 4.3 KB
  • GitHub page: 16.78 ms, 1.81 MB

These benchmarks, conducted using rustc 1.91 on Ryzen 9 9950X, can be found in css-inline/benches/inliner.rs.

Command Line Interface

Installation

Install with cargo:

Usage

The following command inlines CSS in multiple documents in parallel. The resulting files will be saved as inlined.email1.html and inlined.email2.html:

css-inline email1.html email2.html

For full details of the options available, you can use the --help flag:

Further reading

If you're interested in learning how this library was created and how it works internally, check out these articles:

Support

If you have any questions or discussions related to this library, please join our gitter!

License

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