This isn't true. The proposal was discussed many times in the WebPerf working group and went through a TAG review before shipping.

Microsoft made a public statement of support for the API, and Firefox engineers were actively involved in many of the design and implementation discussions.

You can read more details in the Intent to Ship thread here: https://groups.google.com/a/chromium.org/d/msg/Blink-dev/Na7...

Somebody does need to actually implement things before it becomes a standard to work out all the kinks and realities that aren't apparent when you're off architecture astronauting on a whiteboard.