I just hit C-x C-s by habit, while in a TextEdit.app window. The document was promptly saved to disk. I’m in shock. I don’t remember ever setting this up, but this implies something pretty weird about the people working on TextEdit.
I’d known about single-key commands like C-a and C-e, but this means they implemented an entire prefix-key system just to handle Emacs-like keybindings in text widgets.