The problem is compositing with the rest of the web page. In VLC you don't have much complexity on top of or around the video (a few buttons, sometimes subtitle

The problem is compositing with the rest of the web page. In VLC you don't have much complexity on top of or around the video (a few buttons, sometimes subtitles or a volume overlay), whereas around a video in a browser you have THE INTERWEBS (insert scream emoji here), which themselves also go through hardware acceleration. Martin Stransky (the implementer of the feature) explains this in one of the related bugzilla bugs, if you want details.