[PageVisibility] This spec is not optimal for iframes

http://www.w3.org/TR/page-visibility<http://www.w3.org/TR/page-visibility/#introduction>
/

The scope of this spec is short sighted. It does not take into account the
scroll position of the page. Because of this, iframes which are below the
fold will still fire that they are visible when they are in fact not
visible and should not be running processes as though they were. Because of
this, I recommend adding this visibility check to the specification so that
browser vendors can enable developers to write more power friendly and
state aware applications.

Jeffrey Gilbert