Drawing WebGL with different contexts attributes inconsistent

I tried creating a WebGL canvas setting `premultipliedAlpha` and `alpha` to various settings, clearing a color, and then drawing the result to a 2D canvas

https://jsfiddle.net/greggman/setm6axu/

Firefox seems to have a bug if premultiplied alpha is set to false although this issue isn't covered in the WebGL spec directly since it's canvas 2d behavior. Should the WebGL tests have a test like this?

Also note while Safari passes the test it fails to display the WebGL canvas correctly but there is no way from JavaScript's POV to test this.

Start with the linked JSFiddle and reproduce the behavior using different WebGL context attributes, especially alpha and premultipliedAlpha, before drawing to a 2D canvas. Review the WebGL test area to determine whether this behavior is covered and whether a reproducible automated assertion is possible; done means the coverage decision and any test expectations are documented.

Written by the indexing model from the issue text.