There seems to be no provision in the spec for a behavior Firefox and IE (and now WebKit-based browsers, too) have. If window.print() is called during page load, then its action is delayed until loading is finished.
The WebKit bug that changed this most recently is <https://bugs.webkit.org/show_bug.cgi?id=43658> - it has some information about trickier aspects of the behavior. This looks like something that needs to be formally specified. - WBR, Alexey Proskuryakov
