I made extensive use of this, when I found it by accident, in my Winamp skins and GUI programs!

This unlocked some memories. I remember on my system the chroma colour not being green but some very dark shade of grey that was almost black but not really black… something like #010101

I've used that trick as far as Windows XP, playing videos inside 3D models in programs like SketchUp

This was a nice trick to protect text from copying. For instance, student assignments. Students could still use digital camera on CRT display, but 20 years ago cameras were costly and students did not have them. And typing text from scratch was a tedious job. So online served assignments were not shared too fast.

"Nowadays, video rendering is no longer done with overlays."

Darn, I thought this explained why, after upgrading my GPU, videos playing in Chrome have a thin green stripe on their right edge.

Iirc you can also set that "green screen" as wallpaper and have video as desktop background!

i had forgotten about this technique when i was at the excellent https://tnmoc.org recently, looking at their sgi irix exhibit featuring a webcam.

the latency of the camera feed on the crt screen was unbelievable even (especially?) by modern standards!

after a minute of pure wonder i remembered about overlays. still mighty impressive.

I had a Matrox Millenium card with a breakout box for capturing RCA, S-Video, and Cable TV; I'd watch TV on my Windows 98 SE2 computer, which was the craziest thing back then, but I always felt like the green-screen like effect was some kind of mysterious bug that I'd better not mess with, or video capture would break. Windows 98 was barely working on a good day, so it felt like the computer was in the process of failing in a graceful and useful way, so I'd better not push my luck.

Every so often you could get a glimpse of the man behind the curtain, by dragging the window quickly or the drivers stuttering, which would momentarily reveal the green color (or whatever color it was) before the video card resumed doing its thing. Switching between full screen and windowed mode probably also revealed the magic, or starting a game that attempted to grab the video hardware context. And of course sometimes other graphical content would have the exact right shade of color, and have video-displaying pixels.