This is a polyfill for scoped CSS, also known as <style scoped>, a feature that is useful for building simple components (but which was removed from the HTML specification around 2016). Check out a small demo.

Usage

Include the polyfill (via your favourite package manager as style-scoped) and then include a <style scoped> tag:

<script src="node_modules/style-scoped/scoped.min.js"></script>
<div>
  <style scoped>
    h1 { color: red; }
  </style>
  <h1>I'm red</h1>
</div>
<h1>I'm not</h1>

This can help you provide CSS encapsulation for your projects. The library is about ~1.9k minified and gzipped.

Supports

The polyfill supports Firefox, Safari and Chromium-based browsers (and should support IE11 and above). (It requires browsers with WeakMap and MutationObserver).

The code works by applying a custom attribute name, e.g. __scoped_123, to the parent element of the <style scoped> tag, and modifying the style rules to have this as a prefix. If the scoped CSS changes (via .textContent), or its node is moved etc, the style rules are reevaluated.

Config

If window.scopedCSS is defined before the polyfill is loaded, it accepts two options:

• scopedCSS.applyToClass (default false): if true, the polyfill will use a class, not an attribute
• scopedCSS.prefix: a custom prefix for the attribute or class name set on the scope

For example:

<script>
  window.scopedCSS = {applyToClass: true, prefix: '_some_custom_name'};
</script>
<script type="module">
import './node_modules/style-scoped/scoped.min.js';
// or maybe
import 'style-scoped';
</script>

Advanced

As well as scoping regular CSS rules, the polyfill also rewrite :scope. This refers to the parent of the <style scoped> tag, e.g.:

<div><!-- just this div will have a blue background -->
  <style scoped>
    :scope {
      background: blue;
    }
  </style>
</div>

Note that :scope is supported by most modern browsers—but without <style scoped> support, it will match the HTML element and is the same as :root.

⚠️ Rules which use :scope inside another selector (e.g. :is(div:scope)) are not currently supported and will be cleared. If this is actually something you need, I will eat my hat. 🎩

Notes

• The polyfill doesn't operate on all CSS rules: e.g., @keyframes, @font are ignored
• If you depend on cross-domain CSS via @import, this is loaded dynamically with an XHR: so it may take a little while to arrive (see background)

Release

Compile code with Closure Compiler.

// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name scoped.min.js
// ==/ClosureCompiler==

// code here