When an iframe is first created, its window_size is set to None. This is later set by Constellation::handle_iframe_size_message (https://github.com/servo/servo/blob/master/components/constellation/constellation.rs#L1312) in response to a FrameSize event from the compositor, which contains the size calculated by webrender in RenderNotifier::pipeline_size_changed (https://github.com/servo/servo/blob/master/components/compositing/compositor.rs#L334).

This is fine, except that it means script may be able to see a transitory zero value for the calculated height and width, if it queries the window size before webrender has reported.

Reported by @Ms2ger on irc: http://logs.glob.uno/?c=mozilla%23servo&s=24+Nov+2016&e=24+Nov+2016#c568181