On the WPE WebKit backend, context-attributes-alpha-depth-stencil-antialias fails when testing how many bits are available for the RGB channels on a no-alpha WebGL canvas. It expects 8bpp, but the surface is 16-bit in this case (RGB565).
Created attachment 375528 [details]
Created attachment 375691 [details]
Comment on attachment 375691 [details]
Clearing flags on attachment: 375691
Committed r248362: <https://trac.webkit.org/changeset/248362>
All reviewed patches have been landed. Closing bug.