Die HTML-Spec sagt: “For radio buttons, the required attribute is satisfied if any of the radio buttons in the group is selected. Thus […] any of the radio buttons can be checked, not just the one marked as required.”

Verwirrt? Nun, da führt die Spec weiter aus: “To avoid confusion as to whether a radio button group is required or not, authors are encouraged to specify the attribute on all the radio buttons in a group.”

Gut, das hab ich falsch vermutet. Es wird aber nicht schwierig sein andere Beispiele zu finden, in denen JS notwendig ist.