Container Query Polyfill

A tiny polyfill for CSS Container Queries, weighing about 1.6kB brotli’d. It transpiles CSS code on the client-side and implements Container Query functionality using ResizeObserver and MutationObserver.

Usage

Ideally, the polyfill is only loaded if the browser doesn’t support Container Queries natively. In a modern setup with a bundler that uses ES modules, the following snippet should work:

const supportsContainerQueries = "container" in document.documentElement.style;
if (!supportsContainerQueries) {
  import("container-query-polyfill");
}

If you are in a legacy setup (or just want to prototype quickly), there’s also an IIFE version that you can include using a <script> tag:

<script src="https://unpkg.com/container-query-polyfill/cqfill.iife.min.js"></script>

Browser support

The polyfill relies on ResizeObserver, MutationObserver and :is(). Therefore, it should work in all modern browsers, specifically Chrome/Edge 88+, Firefox 78+ and Safari 14+.

Feature support & limitations

My aim is to make the polyfill work correctly for the majority of use-cases, but cut corners where possible to keep the polyfill simple(-ish), small and efficient. The limitations arising from these tradeoffs are listed below.

(These decisions can be revisited if they pose a significant hurdle and there is a good way to implement them. Please open an issue!)

• Both the old CQ syntax as well as the new syntax are supported:

/* These are all equivalent */
@container (min-width: 200px) {
  /* ... */
}
@container (width >= 200px) {
  /* ... */
}
@container size(width >= 200px) {
  /* ... */
}

• Boolean operations (and, or and not) are supported.
• The polyfill does not support style queries (e.g. @container style(--color: red)), as there is no way to get notified of computed style changes.
• The polyfill does not support pseudo elements (::before & ::after), as they don’t have a real DOM handle and can't be observed with ResizeObserver.
• Container Queries will not work when nested inside a Media Query. For now, the polyfill only supports top-level CQs.
• Container Query thresholds can only be specified using pixels.
• Due to the nature of CORS, the polyfill only attempts to handle same-origin and inline stylesheets. Cross-origin stylesheets are not processed, regardless of CORS headers.
• CQs inside ShadowDOM are not supported yet.
• Don’t do weird interspersed comments, okay? Like @container /* here’s a comment! */ (min-width: 1px) { ... }. Just don’t.

Building & Testing

This project uses esbuild to bundle the project, which is automatically installed via npm. To build the polyfill, run:

npm run build

To run the tests, run

npm run serve

and open your browser at http://127.0.0.1:8081/tests.

License Apache-2.0