[whatwg] keepalive attribute on iframe

[Ojan Vafai]

We should add a keepalive attribute to iframes that prevents iframes from
being unloaded/reloaded when removed from or appended to a document.
Similarly, a disconnected iframe with keepalive should load. If the
keepalive attribute is removed from a disconnected iframe, then it should
unload.

I'm not terribly happy with the name 'keepalive', but I can't think of
anything better at the moment.

As iframes increasingly become the standard way of achieving certain tasks
(e.g. sandboxing), it's increasingly important to be able to move them
around in the DOM. Right now, to achieve this sort of keepalive behavior,
you have to keep the iframe always appended to the document and position it
absolutely as the document changes.

Ojan