[html] Issue: "valid email address" doesn't allow I DNs… marked as i18n

chaals has just labeled an issue for https://github.com/w3c/html as 
"i18n":

== "valid email address" doesn't allow IDNs… ==
The definition of a [valid email 
address](https://w3c.github.io/html/sec-forms.html#valid-e-mail-address)
 only allows for ascii letters, digits, and hyphens in domain names, 
and only ascii printable characters before the @ sign. This doesn't 
match reality, which now includes IDNs.

Should we insist on putting punycode everywhere, or should we try to 
hide that from users and specify how to make internationalised email 
addresses?

See https://github.com/w3c/html/issues/538