CSS Anchor Positioning Polyfill

Browser Support

Firefox 54+
Chrome 51+
Edge 79+
Safari 10+

Getting Started

To use the polyfill, add this script tag to your document <head>:

<script type="module">
  if (!("anchorName" in document.documentElement.style)) {
    import("https://unpkg.com/@oddbird/css-anchor-positioning");
  }
</script>

If you want to manually apply the polyfill, you can instead import the polyfill function directly from the @oddbird/css-anchor-positioning/dist/css-anchor-positioning-fn.js file.

For build tools such as Vite, Webpack, and Parcel, that will look like this:

import polyfill from '@oddbird/css-anchor-positioning/fn';

The polyfill function returns a promise that resolves when the polyfill has been applied.

You can view a more complete demo here.

Configuration

The polyfill accepts one argument (type: boolean, default: false), which determines whether anchor calculations should update on every animation frame (e.g. when the anchor element is animated using transforms), in addition to always updating on scroll/resize. While this option is optimized for performance, it should be used sparingly.

<script type="module">
  if (!("anchorName" in document.documentElement.style)) {
    const { default: polyfill } = await import("https://unpkg.com/@oddbird/css-anchor-positioning/dist/css-anchor-positioning-fn.js");

    polyfill(true);
  }
</script>

When using the default version of the polyfill that executes automatically, this option can be set by setting the value of window.UPDATE_ANCHOR_ON_ANIMATION_FRAME.

<script type="module">
  if (!("anchorName" in document.documentElement.style)) {
    window.UPDATE_ANCHOR_ON_ANIMATION_FRAME = true;
    import("https://unpkg.com/@oddbird/css-anchor-positioning");
  }
</script>

Limitations

This polyfill was implemented against an early version of the spec, and updates were paused to allow the syntax to solidify. Now that browsers are working on implementation, we would like to bring it up to date.

While this polyfill supports many basic use cases, it doesn't (yet) support the following features:

The following portions of Position Fallback:
- position-try-order. If try-size is specified in position-try shorthand, it will be parsed, and try-tactics will be applied, but the try-size will be ignored.
- The flip-start try-tactic is only partially supported. The tactic is only applied to property names and anchor sides.
- a position-area as a try-tactic
- Fallback does does not support anchor functions that are nested or passed through custom properties.
Polyfill allows anchoring in scroll more permissively than the spec allows, for instance without a default position-anchor.
anchor-scope property on pseudo-elements
position-area property
anchor-center value for justify-self, align-self, justify-items, and align-items properties
anchor functions with implicit anchor-element
automatic anchor positioning: anchor functions with inside or outside anchor-side
position-visibility property
dynamically added/removed anchors or targets
anchors or targets in the shadow-dom
anchor functions assigned to inset-* properties or inset shorthand property
vertical/rtl writing-modes (partial support)

In addition, JS APIs like CSSPositionTryRule or CSS.supports will not be polyfilled.

Sponsor OddBird's OSS Work

At OddBird, we love contributing to the languages & tools developers rely on. We're currently working on polyfills for new Popover & Anchor Positioning functionality, as well as CSS specifications for functions, mixins, and responsive typography. Help us keep this work sustainable and centered on your needs as a developer! We display sponsor logos and avatars on our website.

Sponsor OddBird's OSS Work