Shadow Container Query Polyfill

This repo is based on https://github.com/GoogleChromeLabs/container-query-polyfill with the added enhancement for shadow dom/web components.

If you find this work helpful please consider buying me a coffee.

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A small (9 kB compressed) polyfill for CSS Container Queries using ResizeObserver and MutationObserver supporting the full @container query syntax:

• Discrete queries (width: 300 and min-width: 300px)
• Range queries (200px < width < 400px and width < 400px)
• Container relative length units (cqw, cqh, cqi, cqb, cqmin, and cqmax) in properties and keyframes

Browser Support

• Firefox 69+
• Chrome 79+
• Edge 79+
• Safari 13.4+

Getting Started

Installation

npm install --save shadow-container-query-polyfill

Alternatively, you can use it directly from a CDN:

<script src="https://cdn.jsdelivr.net/npm/shadow-container-query-polyfill@1/dist/shadow-container-query-polyfill.modern.mjs"></script>

For the best user experience, it's recommended that you initially only use the polyfill for content below-the-fold and use @supports queries to temporarily replace it with a loading indicator until the polyfill is ready to display it:

@supports not (container-type: inline-size) {
  .container,
  footer {
    display: none;
  }

  .loader {
    display: flex;
  }
}

You can view a more complete demo here. On sufficiently fast networks and devices, or devices that natively support Container Queries, this loading indicator will never be displayed.

Note Keep in mind that this technique effectively limits impact on FID and CLS, potentially at the expense of LCP. You may see regressions in the latter as a result, particularly on lower end devices or in poor network conditions.

Limitations

• CSS first: The polyfill currently only supports <style> and same-origin <link> elements. Inline styles via the style attribute or CSSOM methods are not polyfilled. Likewise, JavaScript APIs like CSSContainerRule are not polyfilled, and APIs like CSS.supports() are not monkey-patched.
• Best effort: Style changes that do not lead to observable DOM or layout mutations (e.g. changing font-size in a container without content) may not be detected, or may be detected a frame late on some browsers. Complex sibling CSS selectors aren't supported.
• Currently, there is no support for functions like calc(...) in container conditions. Your contribution would be welcome!

Supporting browsers without :where()

The polyfill uses the CSS :where() pseudo-class to avoid changing the specificity of your rules. This pseudo-class is relatively new, however. If you need to support browsers without it, you will need to append the dummy :not(container-query-polyfill) pseudo-class to the originating element of every selector under a @container block:

Before	After
@container (min-width: 200px) { #foo { /* ... */ } .bar { /* ... */ } #foo, .bar { /* ... */ } ul > li { /* ... */ } ::before { /* ... */ } }	@container (min-width: 200px) { #foo:not(.container-query-polyfill) { /* ... */ } .bar:not(.container-query-polyfill) { /* ... */ } #foo:not(.container-query-polyfill), .bar:not(.container-query-polyfill) { /* ... */ } ul > li:not(.container-query-polyfill) { /* ... */ } :not(.container-query-polyfill)::before { /* ... */ } }

This is to ensure the specificity of your rules never changes (e.g. while the polyfill is loading, or on browsers with native support for container queries). On browsers without :where() supports, rules without the dummy will be ignored.

License

Apache 2.0