http://fullscreen.spec.whatwg.org/#model says:

3. Remove elements from doc's fullscreen element stack until there is one element left.

This doesn't say which elements to remove. WebKit/Blink currently removes all elements except the top element. The spec should do, because:

1. It doesn't cause document.fullscreenElement to change until the fullscreenchange event is fired.

2. If the top element was an iframe (no other element could be), the cross-document chain of fullscreen elements remains intact. Anything else can temporarily leave you with a gap in the chain. Having simple unbroken chains seems like a good invariant, then the implementation can assert it whenever pushing or popping to a fullscreen element stack and maybe prevent some bugs.