When the Metal ANGLE backend is enabled, the values passed to glClearColor have red and blue flipped on Mac. See fast/canvas/webgl/match-page-color-space.html (unfortunately the WebGL conformance suite never assigns different values to the color channels - just greyscale)
<rdar://problem/75120583>
Created attachment 422464 [details] Patch
Wow, can't believe the conformance suite doesn't cover this case. Filed https://github.com/KhronosGroup/WebGL/issues/3254 about improving its coverage.
Comment on attachment 422464 [details] Patch Looks good overall, but would like to see layout test results changing / progressing along with this patch. The BGRA/RGBA difference in Metal is highly unfortunate. I wish it had not exposed BGRA in the first place.
Created attachment 422591 [details] Patch
Note that there are important steps to take when updating ANGLE. See https://trac.webkit.org/wiki/UpdatingANGLE
Comment on attachment 422591 [details] Patch Looks good. The ios-wk2 layout test failures look unrelated. r+
Landed in r274547