[whatwg/url] DNS (and other naming systems) vs the host parser (#397)

@sleevi I was curious if we could explore https://github.com/whatwg/url/issues/159#issuecomment-271674428 further to make a more principled determination about #390 and similar such issues.

Firefox hit a compatibility issue with HTTPS URLs with hyphens at the beginning or end of domain labels and also previously with underscores: https://bugzilla.mozilla.org/show_bug.cgi?id=1184059. It doesn't use this validation for HTTP URLs though. This validation being:

> RFC 5280 Section 4.2.1.6 says that a dNSName "MUST be in the 'preferred name syntax', as specified by Section 3.5 of [RFC1034] and as modified by Section 2.1 of [RFC1123]" except "a dNSName of ' ' MUST NOT be used." Additionally, we allow underscores for compatibility with existing practice.

What I'm interested in is learning which naming systems are still in use and what their theoretical range of values is as well as their range of values in widely adopted implementations.

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