Mechanism, not policy
The design principle behind X11: the server supplies the tools for drawing windows but refuses to decide how they should look or behave — that is left to the clients.
Bob Scheifler and Jim Gettys wrote the rule down in their 1986 paper on X: provide mechanism rather than policy, and in particular put user-interface policy in the clients' hands. The idea was older — the separation of mechanism and policy had been articulated in Carnegie Mellon's Hydra operating system in the 1970s — but X pushed it further than any window system before or since. The server knows how to create windows, draw lines and deliver events. Where a window lands, whether it gets a title bar, what a button looks like or which window has focus is none of its business.
The bill was visible for twenty years. The window manager became a separate, replaceable program, which produced hundreds of them, and the ICCCM was needed in 1989 precisely to make clients agree on what the server had refused to regulate. Motif and OPEN LOOK fought over the look with rival widget sets, and an X desktop looked different on every machine. Wayland, designed by people who had seen the consequences, pulls some policy back into the protocol and merges the window manager with the server. The rule is still cited as both X's greatest strength and its greatest weakness, often in the same sentence.