Removing features from browsers isn't a realistic "solution" if the end result is a "minimal browser" that normal users don't want to use. There are many simpler/minimalist web browsers: https://www.google.com/search?q=minimalist+open+source+brows...
The common issue with all of them is "some websites don't work with it". This means the minimalist browsers create a tiny self-selected group of hardcore technically savvy users because they are they only ones who are willing to put up with random incompatibilities.
Whatever <X features> you want to hypothetically remove from browsers such that an individual coder can "hold the smaller source code all in their head" -- will create a limited and more "broken" browser from the perspective of normal mainstream web surfers.
With the virtualization/containerization facilities baked into common desktop OSes today, we could deliver a complete (and far more secure) experience, needing only a fraction of the boilerplate WHATWG stuffed into Chrome and Firefox over the years.
And with the magic of compilers, we could transpile most existing work down to the new simplified VM. Most “web apps” are already getting transpiled from typescript/javascript to minified javascript. Changing the target is not an insurmountable task.
As for video and audio, they could be done better AND more simply than the typically are at the moment.
25 years ago, on my 1GHz computer, with 512MB of RAM, I had no trouble watching videos, playing music, sending files & emails, making calls, etc--all over the internet. So many people are confused into thinking that computing didn't begin until javascript, which was not the beginning, but the beginning of the end.
Exactly like IE before it, but even more extreme. In IE's case, websites avoided IE extensions because they only worked on IE, even when it was the dominant browser.