This check has been removed from the spec, Firefox and Chrome.
All information here:
Created attachment 347671 [details]
Comment on attachment 347671 [details]
It seems there is a WPT test for it https://github.com/web-platform-tests/wpt/pull/7385 ; should we also import / update the test?
Created attachment 347778 [details]
Comment on attachment 347778 [details]
Clearing flags on attachment: 347778
Committed r235174: <https://trac.webkit.org/changeset/235174>
All reviewed patches have been landed. Closing bug.