The CSS anchor positioning specification defines anchor positioning, "where a positioned element can size and position itself relative to one or more 'anchor elements' elsewhere on the page." This CSS Anchor Positioning Polyfill supports and is based on this specification.
- Firefox 54+
- Chrome 51+
- Edge 79+
- Safari 10+
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.
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>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.
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.
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).
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. Iftry-sizeis specified inposition-tryshorthand, it will be parsed, andtry-tacticswill be applied, but thetry-sizewill be ignored.- The
flip-starttry-tacticis only partially supported. The tactic is only applied to property names and anchor sides. - a
position-areaas atry-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-scopeproperty on pseudo-elementsposition-areapropertyanchor-centervalue forjustify-self,align-self,justify-items, andalign-itemsproperties- automatic anchor positioning: anchor functions with
insideoroutsideanchor-side position-visibilityproperty- dynamically added/removed anchors or targets
- anchors or targets in the shadow-dom
- anchors or targets in constructed stylesheets (#228)
- anchor functions assigned to
inset-*properties orinsetshorthand property - vertical/rtl writing-modes (partial support)
- implicit anchors or the
position-anchor: autokeyword (pending resolution of whatwg/html#9144)
In addition, JS APIs like CSSPositionTryRule or CSS.supports will not be
polyfilled.
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.
