Great Idea, but not working for me. When I install it does nothing. Any ideas why?

I tried it on https://www.crazygames.com/game/uno-online To fullscreen a game, it does nothing, i had mixed results in various other pages, sometimes it does work, sometimes the screen just stays black.
I was hoping to be able to full screen a screen selection of the page, rather than it automatically detecting where boxes are.