As Florian points out, the WebGL 1.0 extensions specs do make strong guarantees. The issue is that some UAs, most notably Chrome, do not support those extensions, for reasons that I can’t recall and am not sure I ever understood fully. As a result they have completely unspecified behavior when it comes to floating-point rendering, leaving developers like Florian grasping for ways to figure out what those UAs are doing. With unspecified behavior there can be no conformance tests, so it is not surprising if things like failure to support RGBA, FLOAT readPixels fall through the cracks.
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