[cssom] getComputedStyle can return null? #572
Description
Related to
#571
and maybe
#219
In Gecko, the following alerts null
<!DOCTYPE html>
<iframe style=display:none srcdoc="<style>body{background:blue}</style>x"></iframe>
<script>
onload = () => {
alert(window[0].getComputedStyle(window[0].document.body))
}
</script>
In WebKit/Chromium/Edge 13 it alerts [object CSSStyleDeclaration].
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4543
cc @bzbarsky