https://html.spec.whatwg.org/multipage/forms.html#valid-e-mail-address

does not take into account EAI, or IDNA.

It should be something like:

email         = local-part "@" host
local-part    = 1*atext2 *("." 1*atext2)
host          = < as defined in https://url.spec.whatwg.org/#host-parsing >

atext2        = atext | utext
atext         = < as defined in http://tools.ietf.org/html/rfc5322#section-3.2.3 >
utext         = XID_Start 1*XID_Continue
XID_Start     = < as defined in http://www.unicode.org/reports/tr31 >
XID_Continue  = < as defined in http://www.unicode.org/reports/tr31 >


Additional conditions: 
 atext2 must be in NFC format, as defined in http://www.unicode.org/reports/tr15

Notes: 
 * for local-part, see dot-atom-text in rfc5322 section 3.2.3
 * the above doesn't provide for quoted email addresses; the syntax would have to be enhanced to allow for those.
 * the restriction to NFC is recommended in http://tools.ietf.org/html/rfc6530#section-10.1, but not required there.
 * the restriction to a Unicode identifier is not in rfc6530, but helps to prevent bizarre email addresses.