[acdha]

It’s only vastly used if you mean “YouTube transcodes all of their H.264 into VP8 so you can hear your CPU fan”. It’s exceedingly rare to see a webm file without a corresponding MP4 file.

[clouddrover]

It depends on the resolution. YouTube stopped offering 4K H.264 encodes about a year ago:

http://www.streamingmedia.com/Articles/News/Online-Video-New...

Apple might never add VP9 support but 4K video from YouTube will work again once YouTube starts encoding to AV1 and Apple adds AV1 support.