Clearly mark conformance checking-only aspects of the IDNA section

[hsivonen]

What is the issue with the URL Standard?

When reading https://url.spec.whatwg.org/#idna in isolation to find out what kind of IDNA library surface the URL Standard needs, it's not apparent what parts of that section are used by User Agents that are not conformance checkers and which are extra parts for conformance checking but not by usual processing.

It would be useful to more clearly mark this in the IDNA section of the spec itself without the reader having to work this out by following the references backwards.

[annevk]

In whatwg/html#10522 we might make beStrct mandatory for email.

The main thing that remains is the ToUnicode algorithm, which thus far hasn't been web-exposed, but is implemented in some fashion (though not necessarily per specification to prevent various confusing code point combinations, even though you can have confusing combinations in ASCII alone as well). I'm not sure what we can authoritatively say about it though until someone actually dives into the various browser algorithms for displaying domain names in Unicode.