Microsoft used to be more proactive. It pressured vendors to have rough feature parity with each other and then released the "harmonized" baseline as the next DirectX version. But as with many things, Microsoft dropped the ball there. The most recent DirectX revision is more than 4 years old, and it doesn't look like there's going to be a new one any time soon.
The only truly unified API is Metal, for obvious reasons.
Agreed, and your had a more easy example at hand, XBox DirectX, versus the PC world.
The point is that many that argue Khronos APIs are portable, don't seem to have deal how portable they actually are in practice, just like POSIX is portable only to certain extent, or Web standards (which then many just ship Chrome with their app).
Microsoft used to be more proactive. It pressured vendors to have rough feature parity with each other and then released the "harmonized" baseline as the next DirectX version. But as with many things, Microsoft dropped the ball there. The most recent DirectX revision is more than 4 years old, and it doesn't look like there's going to be a new one any time soon.
The only truly unified API is Metal, for obvious reasons.