Bugzilla

I wasn't able to narrow down exactly what caused this regression, since it went too far back in time (I got a range of revisions on March 30, 2000), so after looking through those and trying to understand if any affected this, I thought to try out what ended up fixing the problem in mac (per [here](https://bugzilla.mozilla.org/show_bug.cgi?id=1718743#c14)). So I basically copied :morgan's patch, applying it to Windows and.... it fixes the black mask around the buttons (the arrow buttons in the above screenshots are anchor tags, not buttons, interestingly). However, if there's some reason I shouldn't fix the problem this way, I'm open to another approach.

Did a manual test on Windows 11 for youtube and html5 video, and the buttons loko fine in HCM and dark themes (OS and Firefox).

I wasn't able to narrow down exactly what caused this regression, since it went too far back in time (I got a range of revisions on March 30, 2000), so after looking through those and trying to understand if any affected this, I thought to try out what ended up fixing the problem for mac (per [here](https://bugzilla.mozilla.org/show_bug.cgi?id=1718743#c14)). So I basically copied :morgan's patch, applying it to Windows and.... it fixes the black mask around the buttons (the arrow buttons in the above screenshots are anchor tags, not buttons, interestingly). However, if there's some reason I shouldn't fix the problem this way, I'm open to another approach.

Did a manual test on Windows 11 for youtube and html5 video, and the buttons look fine in HCM and dark themes (OS and Firefox).