This is deliberate. If you set display: fullscreen, we explicitly disable the HTML fullscreen API to avoid any clobbering between the two. There are questions of what the back stack should contain, what UI transitions there should be when you're in display: fullscreen and then you make something on the page fullscreen, etc.

The team doesn't really have cycles to explore whether or not we can resolve all of this right now unfortunately.

Does Opera's implementation behave the same? Was this properly documented to web developers? It sounds unfortunate that an API is completely disabled because of how the websites is being launched :(

Fullscreen video playback is a primary video use case. 

Is our recommendation that video sites not use "display" : "fullscreen"?

I don't think that's a viable solution in the long term: two use cases for 'display: fullscreen' are games and VR and they might want to play videos. They might make fake fullscreen by stretching the video to the size of the viewport but that wouldn't be a real future forward solution.

In the (sometime) term, we'll absolutely get the fullscreen API working properly with display: fullscreen. We're just a little stretched at the moment unfortunately. :(

I'm going to be OOO for a week, but I'll try and set aside some time to scope out how complicated this might be when I'm back.

Why clobber? Do we use the same fullscreen activity for content for fullscreen web app? Seems we should have a separate one as it is browser level, not content level.

Over to googleo@ who is aiming to fix this for M61.

Marking as available for now. We don't have eng resources to tackle this in SYD unfortunately.