The page screenshot is taken when the load event fires, but this event does not wait for resources linked to by CSS (such as background-image: url("foo.png"), or @font-face, or the like). This makes testing certain CSS features very difficult. It would be helpful to wait until all such resources have loaded before taking the screenshot. (Note that this will require a separate mechanism, as the load event is specced to not wait for these, at least until w3c/csswg-drafts#1088 gets resolved.)

A hack that can be used for images for now, is to add an <img src="foo.png"> tag to the document, since the load event will wait for those images to load.