Thanks for fixing this! Though: technically we should be fixing this in the layout/reftests/w3c-submitted version (that's the "master copy" in our source tree, and then we do single-directional sync from there to upstream and then bidirectional sync between upstream wpt and our wpt import). Right now, I suspect this patch's changes will just get overwritten unless you include the layout/reftests copy as well.

So specifically, the file that needs fixing here is: https://searchfox.org/mozilla-central/source/layout/reftests/w3c-css/submitted/contain/contain-layout-suppress-baseline-002.html#9 Ideally we should make this fix in *only* that version of the test, and allow the normal sync pipeline to play itself out (and I think dbaron syncs that to upstream wpt ~weekly). If you want to fix both in-tree copies (as you did in bug 1491235), that's OK too, but slightly less preferred - it just adds a small bit of overhead when dbaron does the sync, I think.