This is more or less the same issue as https://www.w3.org/Bugs/Public/show_bug.cgi?id=15489 but I think it's worth another look since a lot of things have changed.

The issue is that the e-mail address validation pattern in sec 4.10.5.1.5 only accepts ASCII addresses, not EAI addresses. Since last time, large hosted mail systems including Gmail, Hotmail/Outlook, Yahoo/AOL (soon if not yet), and Coremail handle EAI mail. On smaller systems Postfix and Exim have EAI support enabled by a configuration flag.

On the other side, writing a Javascript pattern to validate EAI addresses has gotten a lot easier since JS now has Unicode character class patterns like /(\p{L}|\p{N})+/u which matches a string of letters and digits for a Unicode version of letters and digits.

Last time around the consensus seemed to be that EAI input fields should be marked as unicode or eai or the like, since it'll be a while since all mail systems handle EAI.

For the avoidance of doubt, when I say EAI, I mean both Unicode local parts and Unicode domain names, since that's what EAI mail systems handle. There is no benefit to translating IDNs to A-labels (the ones with punycode) since that's all handled deep inside the mail system.