The buggy versions that do not follow the spec actually make it possible to use clamp(uv, srcOriginStart, srcOriginEnd) in the C3 predraw case.
The versions that follow the WebGL spec do not work with the C3 predraw case where srcOriginStart.y > srcOriginEnd.y. The webgl spec is: min(max(x, minVal), maxVal).
So, if the bugs were fixed to match the OpenGL spec, then clamp(uv, srcOriginStart, srcOriginEnd) could not be used with C3 in the predraw case on any platform.
I think to handle the variable C3 behavior between predraw and no predraw, clamp and platform variance, for effect distortions where vTex is changed, I will use must predraw and also my own definition of clamp.