IDNA: avoid defining valid domain string in terms of the parser

[annevk]

This is basically something we need to raise again with the IDNA folks as their document does not really address it. This used to be tracked by https://www.w3.org/Bugs/Public/show_bug.cgi?id=25334.

As part of fixing this we should make it clear they are at least ASCII case-insensitive.

[dubzzz]

Maybe I'm wrong but aren't valid domains defined in the RFCs below?

• https://www.ietf.org/rfc/rfc1034.txt
• https://www.ietf.org/rfc/rfc1123.txt

The first one saying:

<domain> ::= <subdomain> | " "
<subdomain> ::= <label> | <subdomain> "." <label>
<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig-hyp> ::= <let-dig> | "-"
<let-dig> ::= <letter> | <digit>

The second one somehow saying that we can start a domain with a figure.

[annevk]

It's not entirely wrong, but those definitions don't account for IDNA and also don't seem to account for ASCII code points that happen to work in practice, such as _. What we want is a definition that does account for that, for which, when the host parser defined in the URL standard is applied to it, the output is not failure.