x-moz-errormessage attribute can be used to have a custom error message
when validating a form field. This is a mozilla-only attribute which was
rejected in 2010 by w3: https://www.w3.org/Bugs/Public/show_bug.cgi?id=10923

Currently, a similar message can be set using setCustomValidity() method:

I intent to remove the support for x-moz-errormessage attribute in 66,
without any deprecation message because it's unlikely that it would break
websites. Let me know if you disagree.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1513890
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