Priority Hints - fetchpriority and iFrames

Would anyone be particularly concerned if we removed iFrames from the list
of elements that support fetchpriority?  It's not currently implemented in
Chrome and as we go through the spec process, it isn't immediately clear
that trying to prioritize the document fetch is the best way to control the
scheduling of the iFrame.

Specifically:

- The fetch will be in a different document context from main-document
subresources and may not be able to be scheduled within the browser
relative to the other requests.

- The network fetch may not be on a connection where it could be
prioritized against other requests (even if they happen to be from the same
origin) depending on how browsers choose to partition the network state
across frames for privacy.

- It doesn't control the scheduling of non-fetch iFrames.

I'm thinking that we'd be better off thinking about scheduling of iFrame
instantiation as it's own thing with imperatives that match what dev's
would like to control.  i.e. would a "defer" make sense for an iframe to
delay starting the loading of an iframe until the main DOM is ready?  Kind
of like there's already loading=lazy for matching the image behavior.