It does try to reduce what it can in low memory situations (less "pre" optimizations like prefetching, prerendering, even dns prefetches and stuff, V8 gets more eager to throw out "potentially unneeded" code, tabs get suspended and written to disk, heuristics can get shut off, etc...), but it can't change what the page does.

If a browser tab wants to store 300MB of data in RAM, or wants to throw a shitton of stuff at the DOM, Chrome needs to oblige.

Nope - it's fairly happy to choke everything else or even crash (tabs or the browser) when RAM gets too scarce. It does seem to have some capability to restrict usage, but it's not thorough enough to avoid performance/crashing issues.

Not in my experience.

With 4 GB ram, Chrome will happily give ~2GB to any tab that requests it in a couple of seconds, leaving me with an unresponsive Windows aggressively paging memory to disk, only to give whatever it finds... back to chrome

Firefox on Linux used to many years ago, it even included swap space as available memory.