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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
styleandlinktags - Removes
styleandlinktags - 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-inlineUsage
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 emailNote 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:truekeepStyleTags. Specifies whether to keep "style" tags after inlining. Default:falsekeepLinkTags. Specifies whether to keep "link" tags after inlining. Default:falsebaseUrl. The base URL used to resolve relative URLs. If you'd like to load stylesheets from your filesystem, use thefile://scheme. Default:nullloadRemoteStylesheets. Specifies whether remote stylesheets should be loaded. Default:truecache. Specifies caching options for external stylesheets (for example,{size: 5}). Default:nullextraCss. Extra CSS to be inlined. Default:nullpreallocateNodeCapacity. 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.