In bug 197755 a preliminary ANGLE backend was added to WebCore's WebGL implementation which doesn't render anything yet. The back buffer needs to be hooked up on the macOS port as a start.
Created attachment 373816 [details]
Note that there are important steps to take when updating ANGLE. See http://trac.webkit.org/wiki/UpdatingANGLE
The attached patch fixes many bugs encountered along the way toward getting the ANGLE backend for WebKit's WebGL implementation to draw correctly to the screen on desktop macOS. It runs the San Angeles demo correctly, as well as parts of the WebGL 1.0 conformance suite.
If necessary the patch can be split up, though this will be a bit difficult since the intermediate patches can't be tested until all of them land.
Comment on attachment 373816 [details]
Clearing flags on attachment: 373816
Committed r247315: <https://trac.webkit.org/changeset/247315>
All reviewed patches have been landed. Closing bug.