# Container Query Polyfill

*Please note that this polyfill is now in maintenance mode, as of Nov, 2022. We are not planning to add more features or enhancements.*
____________________________

A small (9 kB compressed) polyfill for CSS Container Queries using [`ResizeObserver`](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver) and [`MutationObserver`](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver) supporting the full [`@container`](https://drafts.csswg.org/css-contain-3/) 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

```bash
npm install --save container-query-polyfill
```

Alternatively, you can use it directly from a CDN:

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

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

  .loader {
    display: flex;
  }
}
```

You can view a more complete demo [here](https://codesandbox.io/s/smoosh-glitter-m2ub4w?file=/index.html). 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](https://github.com/GoogleChromeLabs/container-query-polyfill/issues/56) aren't supported.
- Currently, there is no support for Shadow DOM, or functions like `calc(...)` in container conditions. Your contribution would be welcome!

## Supporting browsers without `:where()`

The polyfill uses the CSS [`:where()`](https://developer.mozilla.org/en-US/docs/Web/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:

<table>
<tr>
<td> Before </td> <td> After </td>
</tr>
<tr>
<td>

```css
@container (min-width: 200px) {
  #foo {
    /* ... */
  }

  .bar {
    /* ... */
  }

  #foo,
  .bar {
    /* ... */
  }

  ul > li {
    /* ... */
  }

  ::before {
    /* ... */
  }
}
```

</td>
<td>

```css
@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 {
    /* ... */
  }
}
```

</td>
</tr>
</table>

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.

## ResizeObserver Loop Errors

When using the polyfill, you may observe reports of errors like `ResizeObserver loop completed with undelivered notifications` or `ResizeObserver loop limit exceeded`. These are expected, and may safely be ignored.

## License

[Apache 2.0](LICENSE)