Created attachment 475522 [details] Screenshot of padding: 100% using the original containing block, and not the containing block generated by the position-area If I use a percentage value for padding on an element that is positioned with `position-area`, the value is computed based on the original containing block, not on the new containing block created by `position-area`. For example, if the anchor is `200px`, and the `position-area` is `center top`, the new containing block should be 200px wide, so I expect `padding-inline-start: 100%` to add 200px of padding. Instead, it uses the original containing block's width. In this example, the containing block is 400px. https://codepen.io/jamessw/pen/KwpZjyj Using Release 220 (Safari 18.4, WebKit 20622.1.14.5), the padding is 400px. In Chromium browsers, the padding is 200px. This matches my understanding of the spec. `margin` appears to be computed based on the correct containing block.