103 Early Hints

Hi folks,

RFC 8297, defining 103 Early Hints was published in 2017. It's been a bit
of a sleeper hit, in the last 18 months or so we've seen uptake and
deployment on client and server sides.

As is natural, we've been gaining experience through deployment. Helping to
identify the areas with Early Hints helps, and areas where there might be
some possible tweaks. One example is that it isn't always useful to emit a
103 Early Hint in response to every request that is received, because the
client's processing context would ignore it.

Client Hints (RFC 8942) has some text that deals with considerations we are
now learning about Early Hints. For instance, a server could emit an
Accept-CH header, and Section 5 of RFC 8942 describes considerations for
the cost of sending Client Hints.

After some chatter on Twitter the past week, a few different people
suggested that something like an Accept-EH request header field might be
useful to help clients to indicate when Early Hints are useful or not. If
we made this a list of field names, it could allow some tailoring of the
emission and content of the hints.

My thinking was maybe its time to upgrade Early Hints from experimental and
roll in some of the learnings / proposals into the update document.

Thoughts?

Cheers,
Lucas