Thanks for a clear, concise, and, if I may say, valuable document.  I couldn't even find any typos.

I have two very minor comments, both of which can be ignored if you prefer:

1. In Section 4, you use "somename.alt" -- was that chosen over "example.alt" for a particular reason?  If so, carry on.  If not, maybe consider switching?

2. I would promote RFC 8244 to a normative reference.  I realize that one can make a reasonable argument both ways, so no big thing either way.

-- 
Barry