focus events on the document

http://www.w3.org/TR/DOM-Level-3-Events/#events-focusevent-doc-focus doesn't
say whether to fire focus/blur-related events on the
documentt/documentElement/body if nothing inside the document is focusable.

Relevant WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=83566.
Test case: https://bug-83566-attachments.webkit.org/attachment.cgi?id=136545
.

There are three cases:
1. No editable content.
2. contentEditable (i.e. focusable) body element.
3. contentEditable (i.e. focusable) html element.

In all browsers, in all three test cases, if you click on the
documentElement or on the body element, document.activeElement points to
the body element (and in the contentEditable cases, you get a blinking
cursor inside the body element). Given that, I think focus events should
fire on the document, documentElement and body elements in all three cases.
This is the IE9+ behavior.

-WebKit only does this if you click on a focusable element in the frame.
-Opera does the WebKit behavior, except doesn't fire focusin/focus in some
cases (looks like a bug since focusout/blur are fired).
-Gecko fires only on the document itself if there are no focusable
elements, but otherwise, does the IE behavior (Gecko only ever fires
focus/blur though, no focusin/focusout).

On the other hand, if script explicitly focuses the documentElement, then
we should only fire focus-related events on the documentElement and the
document, since document.activeElement actually points to the
documentElement in that case.

In short, to keep the platform sane, I think focus-related events should
match how we set document.activeElement.

Curious to hear what other browser vendors think before we change WebKit to
match IE.

Ojan