Most of the RAM & CPU resources used by web browsers are mainly the fault of web developers who have no clue about memory management and will happily write code that uses 100% of a CPU core to do nothing.
The vast majority of websites could be plain text, throw out the scripts.
That's mostly true. If you use a script blocker, most websites work fine, and the ones that don't usually only need one or two scripts enabled out of dozens of running blocked scripts.