The model update landed at https://github.com/mozilla-services/fathom-login-forms/commit/368cbce234fcd2804f82d88d10ac940b8243c496 will fix this, even if the site never corrects their errant markup. Yay!
Bug 1629892 Comment 2 Edit History
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Double-checked, and the model update landed at https://github.com/mozilla-services/fathom-login-forms/commit/368cbce234fcd2804f82d88d10ac940b8243c496 does call this a negative.