Still need more tests. Added a comment to provide clarity around
@HTML5Number and the like. It appears that they are more-or-less useless
at this point because ValidityState#typeMismatch is never set to "true"
for these types. This is because the browser won't even let you enter
    invalid values for those fields. The WHATWG and W3C specs also say
    that this value is only set to true for the email and url types.