Now someone needs to start a project to find a large, documented subset for which ANGLE(ToGL(x)) = x so we can all get rid of this historical two-standards-that-do-exactly-the-same-thing nonsense.

All that reminds me of is this: https://xkcd.com/927/

Yes, one must be careful not to define a new standard, but rather to observe a strict subset of both for which that "equation" holds.

Relevant XKCD: http://dynamic.xkcd.com/random/comic/

Only for WebGL, though. (Edit: OpenGL ES 2)

Actually it's up to OpenGL ES 3 (which may or may not be good enough for your app)

WebGL is just an OpenGL ES 2 binding to JavaScript. OpenGL ES 2/3 is a subset of OpenGL 3/4. Angle translate GLSL shaders and OpenGL calls to DirectX

Sadly it is a bit more than "just", specially if one needs to write code across plain GL and GL ES.

What hurts the most? I would think the imperative drawing stuff would be missed least.

Which OpenGL flavor to support 2.x, 3.x or 4.x, followed by which GLSL level to support, and which GL ES flavor to support 1.x, 2.0 or 3.0 and their respective GLSL levels.

Not to mention vendor specific extensions and workarounds driver bugs.

So one ends with multiple code paths, if several versions are to be supported. Similar to how HTML or POSIX are portable, but then again not so much.