This is an authoring requirement, not a browser implementation requirement. In practice, it means conformance checkers (aka validators) should error for lone radio buttons.
From: Mikko Rantalainen <mikko.rantalai...@peda.net> Sent: Apr 7, 2016 7:49 AM To: firstname.lastname@example.org Subject: [whatwg] Document with a single input[type=radio]? The spec says in 18.104.22.168.16 Radio Button state (type=radio) https://html.spec.whatwg.org/multipage/forms.html#radio-button-state-%28type=radio%29 "A document must not contain an input element whose radio button group contains only that element." What this is supposed to mean in practice? Could this sentence be dropped because this does not match real world browser behavior? (A streaming browser will may hit this case after parsing the first radio button element in the document. Then what?) -- Mikko