If GTK can't do hardware acceleration (which is quite unlikely) it's better to to just disable acceleration in Webkit too, instead of using the glReadPixels fallback solution that is slow and mostly untested.
Pull request: https://github.com/WebKit/WebKit/pull/10928
Committed 261126@main (c1109bfc0991): <https://commits.webkit.org/261126@main> Reviewed commits have been landed. Closing PR #10928 and removing active labels.