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CSS Anchor Positioning Polyfill

Build Status

Netlify Status

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 supports a small number of options. When using the default version of the polyfill that executes automatically, options can be set by setting the value of window.ANCHOR_POSITIONING_POLYFILL_OPTIONS.

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

When manually applying the polyfill, options can be set by passing an object as an argument.

<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({
      elements: undefined,
      excludeInlineStyles: false,
      useAnimationFrame: false,
    });
  }
</script>

elements

type: HTMLElements[], default: undefined

If set, the polyfill will only be applied to the specified elements instead of to all styles. Any specified <link> and <style> elements will be polyfilled. By default, all inline styles in the document will also be polyfilled, but if excludeInlineStyles is true, only inline styles on specified elements will be polyfilled.

excludeInlineStyles

type: boolean, default: false

When not defined or set to false, the polyfill will be applied to all elements that have eligible inline styles, regardless of whether the elements option is defined. When set to true, elements with eligible inline styles listed in the elements option will still be polyfilled, but no other elements in the document will be implicitly polyfilled.

useAnimationFrame

type: boolean, default: false

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.

For legacy support, this option can also be set by setting the value of window.UPDATE_ANCHOR_ON_ANIMATION_FRAME, or, when applying the polyfill manually, by passing a single boolean with polyfill(true).

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 are in the process of bringing 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 not support percentage anchor-side values, nor anchor functions that are 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