Created attachment 460214 [details] testcase 1 STR: 1. Load attached testcase. 2. Hover the orange area. ACTUAL RESULTS: The orange area's width suddenly jumps to twice its original width. EXPECTED RESULTS: The orange area's width should grow gradually (in response to the font-size transition). NOTES: The element here has a width specified in `em` units, and hovering triggers a change to `font-size` which triggers a CSS transition. That should add a value for `font-size` to the cascade, per https://www.w3.org/TR/css-transitions-1/#application , which should in turn influence the resolution of `em`-valued properties. Safari 15.5 gives ACTUAL RESULTS. Firefox and Chrome give EXPECTED RESULTS.
Note: I ran across this bug due to a bug report in Firefox, which turned out to be the reporter inadvertently depending on this WebKit bug. (Chrome matches Firefox's behavior FWIW.) That report was https://bugzilla.mozilla.org/show_bug.cgi?id=1774014, which I closed as invalid since Firefox and Chrome are pretty-clearly following the spec. WebKit happens to match the reporter's expectations over there, but I'm pretty sure that's just an accident and is essentially due to this bug here. If/when you fix this bug, I suspect you'll match Firefox and Chrome on the jsfiddle and attached testcase on https://bugzilla.mozilla.org/show_bug.cgi?id=1774014 .
<rdar://problem/95105214>
Thanks for filing this Daniel. Do you know whether this is already covered by WPT?
I think this might be covered by the css/css-transitions/transition-base-response-*.html tests that WebKit is the sole engine to fail :(
Created attachment 460235 [details] CSS Animation of font-size combined with relative width value Adding another test where we use a CSS Animation to animate font-size. In that example, the em-based width is never updated.
(In reply to Antoine Quint from comment #3) > Thanks for filing this Daniel. Thanks for taking a look! > Do you know whether this is already covered > by WPT? I don't know offhand, no (though it sounds like you might've subsequently found some related WPT tests).
This was fixed in <https://commits.webkit.org/258514@main> *** This bug has been marked as a duplicate of bug 210963 ***