[cssom] getComputedStyle only supports ::before/::after #2149
Description
https://drafts.csswg.org/cssom/#extensions-to-the-window-interface
Roughly, from what I've tested:
- Chrome, Firefox, and Safari all support gCS
::first-letterand::first-line. - Firefox throws
NS_ERROR_NOT_AVAILABLEfor::marker(and doesn't support::markerAFAIK), Chrome doesn't support it so returns the element, and Safari does support::markerand does support it for gCS - Firefox supports gCS
::placeholder, Chrome and Safari does not (despite supporting the feature)
Is there any reason not to allow all pseudo-elements for gCS?