According to spec (https://drafts.csswg.org/cssom-view/#dom-mouseevent-offsetx), offsetX/offsetY should return the values relative to the element's padding edge. Firefox does this correctly, WebKit (and also Chromium, currently), return the value relative to the nearest block ancestor. See https://codepen.io/noamr-the-selector/pen/WNxRbmZ
Corresponding Chromium bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1141782
Relevant WPT issue: https://github.com/web-platform-tests/wpt/issues/15089
<rdar://problem/70857665>