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Updates to the Anchor Position Polyfill

Catching up to the spec

Our sponsors are supporting the continued development of the CSS Anchor Positioning Polyfill. Here’s a summary of the latest updates.

A few years ago, OddBird started working on a polyfill for CSS Anchor Positioning as the spec started to take form. Now that the spec has stabilized and Chromium has begun implementation, we are relying on our sponsors to support development of the polyfill. Thanks to their contributions, we’re releasing v0.1.0.

  1. position-anchor

In their full form, anchor functions can be quite verbose and repetitive.

#target {
  top: anchor(--my-anchor bottom);
  width: anchor-size(--my-anchor width);

position-anchor allows you to specify a default anchor for an element.

#target {
  position-anchor: --my-anchor;
  top: anchor(bottom);
  width: anchor-size(width);

You can also use this to share positioning rules while using different anchors. Both Target A and Target B will be positioned to the right of their anchor, but they will use different respective anchors.

.target {
  left: anchor(right);
.target#a {
  position-anchor: --anchor-a;
.target#b {
  position-anchor: --anchor-b;

position-anchor will also be useful for things like inset-area, which isn’t yet supported by the polyfill.

  1. Anchor validity

Thanks to a great contribution from @ayoreis, the validity algorithm has been updated to match the spec. More can be found in the issue, but this change makes more elements available for anchoring. Notably, you can now anchor to another element that is anchored as well (as long as the anchor element is layed out before the target element).

  1. Support multiple names in anchor-name.

In this example, both #target-a and #target-b are referring to the same element, but with different names.

.anchor {
  anchor-name: --a, --b;
#target-a {
  position-anchor: --a;
#target-b {
  position-anchor: --b;

This will enable a number of use cases where a target can be anchored to different anchors, depending on what is available. Perhaps some of your pages have a different layout, where you want #target-b to anchor to a #side-anchor if there is one, but otherwise to the default .anchor.

.anchor#side-anchor {
  anchor-name: --b;

While a lot of the basic functionality is already possible with the polyfill, there’s a lot left to do to bring the polyfill up to date with the spec. Our v1.0.0 Milestone is prioritized based on what we think will be the most impactful and useful features, and some of the upcoming ones are:

If you’re as excited as I am to use these features, there are a few ways you can help. First, let us know which features you’re most eager to see supported by commenting on the GitHub issues. We also welcome PRs.

And of course, sponsoring OddBird’s Open Source work is a great way to help make our continued work on this polyfill (and others) possible. We’re grateful for our existing sponsors who have made this release possible, and you can see them below.

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