<input type=radio name=group1 required id=radio1>
<input type=radio name=group1 id=radio2>
radio2 is always valid in WebKit because it has no "required". This is a behavior described in old HTML5 draft.
The current HTML specification says radio2 should be invalid because radio1 has required and there is no checked radio button in the group.
The behavior was fixed by http://trac.webkit.org/changeset/105710