The spec has this to say about anchor-center (https://drafts.csswg.org/css-anchor-position-1/#anchor-center): > When using anchor-center, by default if the anchor is too close to the edge of the box’s original containing block, it will “shift” from being purely centered, in order to remain within the original containing block. It appears WebKit doesn't implement this behavior. Steps to reproduce: * Open this demo: https://codepen.io/yisi/details/yLrjKEO * Check the "anchor-center" checkbox * Move the infobox to leftmost and right most position. Observe that in Chrome, the infobox shifts to the right when it's close to the left edge, and shifts to the left when it's close to the right edge. WebKit doesn't do this.