The original Mac OS X dock supported "live" icons. If you minimized the System Monitor, the icon turned into a real time CPU graph.
If you minimized a QuickTime video, the video continued playing, tiny, right in the dock.
I was really sad when this went away, and I don't really understand why it did.
On macOS Mojave, the Activity Monitor still does show real-time graph in the icon if you want it to - right-clicking the icon gives you various graphs to choose from.
Don't know about the QT videos though.
I've gotten to the point where I don't even use the dock anymore, it just stays hidden. Quite a long way from the good ole' days of themes.org, freshmeat.net, and spending hours customizing my desktop environment.