Should be covered by http://caniuse.com/#feat=form-validation
Perhaps searching for setCustomValidity should match http://caniuse.com/#feat=form-validation.
Also, this isn't the same. setCustomValidity is about changing the browser's default field validation error message.

I think this should be listed in HTML5 form features Sub-features
Available at https://caniuse.com/#feat=mdn-api_htmlinputelement_setcustomvalidity
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Perhaps searching for
setCustomValidityshould match http://caniuse.com/#feat=form-validation.