The mhtml tests seem to randomly fail on both GTK and WPE, with the failing case having entirely blank images. Maybe there is some kind of race condition in the test that doesn't wait for something to finish loading?
Created attachment 434908 [details] Patch
Committed r280637 (240250@main): <https://commits.webkit.org/240250@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 434908 [details].
<rdar://problem/81521649>
Reopening to track the fix.