Actually, they do, sort of. At least, if you use GLSL shaders. I remember some demo project using it in a way that made it appear that the website "sees" another window with the same website, which blew my mind when I saw it first. Cannot recall the name or anything else to locate it.

My guess is that you’re remembering this demo: https://xcancel.com/_nonfigurativ_/status/172732259457002734...