(Just a note as I don't have time to add it right now)

During Steam Games one can open a complete web browser in the game overlay. The user agent is

Mozilla/5.0 (Linux; U; X11; en-US; Valve Steam GameOverlay/1586022601; ) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.117 Safari/537.36