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:

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

into:

  
  
    <h1 style="color:blue;">Big Text</h1>
  

• 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
• Tested on Node.js 18 & 20.

Playground

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

Installation

Node.js

Install with npm:

npm i @css-inline/css-inline

Usage

import { inline } from "@css-inline/css-inline";

var inlined = inline(
  `
  
    
      <style>h1 { color:red }</style>
    
    
      <h1>Test</h1>
    
  
  `,
);
// Do something with the inlined HTML, e.g. send an email

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:

import { inlineFragment } from "@css-inline/css-inline";

var inlined = inlineFragment(
  `
  <main>
    <h1>Hello</h1>
    <section>
      <p>who am i</p>
    </section>
  </main>
  `,
  `
  p {
      color: red;
  }

  h1 {
      color: blue;
  }
  `
);
// HTML becomes this:
// <main>
// <h1 style="color: blue;">Hello</h1>
// <section>
// <p style="color: red;">who am i</p>
// </section>
// </main>

Configuration

• inlineStyleTags. Specifies whether to inline CSS from "style" tags. Default: true
• keepStyleTags. Specifies whether to keep "style" tags after inlining. Default: false
• keepLinkTags. Specifies whether to keep "link" tags after inlining. Default: false
• baseUrl. The base URL used to resolve relative URLs. If you'd like to load stylesheets from your filesystem, use the file:// scheme. Default: null
• loadRemoteStylesheets. Specifies whether remote stylesheets should be loaded. Default: true
• cache. Specifies caching options for external stylesheets (for example, {size: 5}). Default: null
• extraCss. Extra CSS to be inlined. Default: null
• preallocateNodeCapacity. 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:

    <style>h1 { color:blue; }</style>


    
    <h1 data-css-inline="ignore">Big Text</h1>

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

  
  <style data-css-inline="ignore">h1 { color:blue; }</style>


  <h1>Big Text</h1>

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:

  
  <style data-css-inline="keep">h1 { color:blue; }</style>


  <h1>Big Text</h1>

Such tags will be kept in the resulting HTML even if the keep_style_tags option is set to false.

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

import { inline } from "@css-inline/css-inline";

var inlined = inline(
  `
  
    
      
      <style>h1 { color:red }</style>
    
    
      <h1>Test</h1>
    
  
  `,
  { cache: { size: 5 } },
);

Caching is disabled by default.

WebAssembly

css-inline also ships a WebAssembly module built with wasm-bindgen to run in browsers.

NOTE: WASM module currently lacks support for fetching stylesheets from network or filesystem and caching.

Performance

css-inline is powered by efficient tooling from Mozilla's Servo project and significantly outperforms other JavaScript alternatives in terms of speed. Most of the time it achieves over a 3x speed advantage compared to the next fastest alternative.

Here is the performance comparison:

The "Basic" case was obtained from benchmarking the example from the Usage section.

The benchmarking code is available in the benches/bench.ts file. The benchmarks were conducted using the stable rustc 1.77.1 on Node.js v21.1.0.

License

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