On the scope of radio buttons in HTML

Question

Several radio buttons with the same name act as a set, where checking one unchecks the others. What is the scope of this behavior?

1. The form in which the button resides
2. The page / document on which the button resides
3. Does scope pass into iframes?

I have always used them in forms, but now writing formless HTML (using ajax for posting), and everything seems to be working just fine, so my guess is #2.

Answer 1

Radio buttons with the same name in the same form act as a set, but not with those of different forms. Radio buttons with no form act as a set with those with no forms.

test it yourself: http://jsfiddle.net/8qqNC/1/

Answer 2

Radio buttons are scoped to the form they are in.

Frames contain external documents, and nothing in them is considered part of the current document, let alone an element within that document.