Not only do you have to support an insane amount of standards, you need to do it fast. Firefox and Chromium are optimized so much for speed, and nobody will use your web browser if it's slow or uses up tons of ram.
Well, they actually can, but it is not magic, it might not work everywhere, but it indeed involves going against website demands. There is reader view in firefox (that parses a page and gives text and images), there's ublock-origin that alone blocks so much adds and tracking that webpages load faster, there is ''i dont care about cookies'' (that automatically selects the cookie options on your chosen option), etc, stuff that could be implemented in the browser as options for the user just like privacy settings.
No, but it can take more than the website demands.
For example, browsers these days pre-load links before you click on them. If I remember correctly (and if it's still implemented like this), Firefox by default would only do a DNS lookup and TLS handshake, to keep memory+power usage and security concerns low, whereas Chrome optimized more for being as fast as possible, and already downloaded the first webpage files.
About a decade ago, I heard from a Chrome developer that their statistics showed that over 90% of users never used the multiple tabs feature. I was shocked at the time, but I'd be even more shocked now.
That said, users do seem to fall into two categories: single tab or a gazillion tabs, with no in between.
Me with my youtube page, then my page with tumblr, twitter and skindeep all in my personal desktop seem very organized and all, until I switch to my university desktop and oh my god why do I still have this article open I've done that presentation LAST SEMESTER oh god I have 30 tabs in just this window and I have 5 more windows with fuck knows how many other tabs!!! Then my third desktop is my torrenting one with some search pages open so that I can just refresh the search for a new episode of whatever series/anime I'm watching now, totaling 1 tab in one single window
I somehow fit perfectly in both extremes and the middle ground lol
I imagine the Chromium devs have put a lot of work into reducing memory usage. Work that'll have to be replicated by whichever small team is working on this hypothetical browser.