@css-inline/css-inline

v0.14.3

Published

7 days ago

High-performance library for inlining CSS into HTML 'style' attributes

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css html email stylesheet inlining

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
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(
  `
  <html>
    <head>
      <style>h1 { color:red }</style>
    </head>
    <body>
      <h1>Test</h1>
    </body>
  </html>
  `,
);
// 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:

<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:

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

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(
  `
  <html>
    <head>
      <link href="http://127.0.0.1:1234/external.css" rel="stylesheet">
      <style>h1 { color:red }</style>
    </head>
    <body>
      <h1>Test</h1>
    </body>
  </html>
  `,
  { cache: { size: 5 } },
);

Caching is disabled by default.

WebAssembly

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

<script src="https://unpkg.com/@css-inline/css-inline-wasm"></script>
<script>
    // Initialize the WASM module first
    cssInline.initWasm(fetch('https://unpkg.com/@css-inline/css-inline-wasm/index_bg.wasm'));

    const inlinedHtml = cssInline.inline(`<html>
  <head>
    <style>h1 { color:blue; }</style>
  </head>
  <body>
    <h1>Big Text</h1>
  </body>
</html>`);

    document.getElementById('output').src = inlinedHtml
</script>

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:

| | Size | css-inline 0.14.0 | css-inline-wasm 0.13.0 | juice 10.0.0 | inline-css 4.0.2 | |-------------|---------|---------------------|--------------------------|-----------------------|----------------------| | Basic | 230 B | 16.82 µs | 20.92 µs (1.24x) | 57.00 µs (3.38x) | 68.43 µs (4.06x) | | Realistic-1 | 8.58 KB | 351.74 µs | 452.28 µs (1.28x) | 859.10 µs (2.44x) | 1.04 ms (2.98x) | | Realistic-2 | 4.3 KB | 203.25 µs | 269.54 µs (1.32x) | 1.02 ms (5.04x) | 861.32 µs (4.23x) |

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.

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