It would be very bad if OS memory usage did not return to flat no?
Btw. many people don't seem to know, that also in languages with a garbage collector like Javascript, you can create awesome memory leaks. And I would bet, most websites actually do: it only works, like it works, because websites are closed regulary. And because RAM is every increasing. But browse with a older smartphone and you hit the RAM limit very quickly.
But that could pile up quickly, so I am not sure if and how it is done in the various browsers.
Not quite. Some leaks are across processes. If your process talk to a local daemon and cause it to hold memory then quitting the client process wont necessarily free it. In a similar way, some application are multi-process and keep some background process active even when you quit to "start faster next time" (of act as spywares). This includes some infamous things like Apple updater that came with iTunes on Windows. It's also possible to cause SHM enabled caches to leak quite easily. Finally, the kernel caches as much as it can (file system content, libraries, etc) in case it is reused. That caching can push "real process memory" into the swap.
So quitting a process does not always restore the total amount of available memory.