Do you know of any forks or clean implementations of browsers which cut out legacy support more aggressively and/or are tuned for performance? Something like Chrome with less overhead because it doesn't bother to support deprecated features.
Dillo parses a ridiculously tiny subset of HTML and CSS, and I used it to browse the Web between 2012 and 2014 when my main workstation was an 800MHz Duron. Yes, I used it as my main browser for two years. Yes, I was using a 19 year old computer 2-4 years ago. :P
Its main issue was that it would crash at inopportune times :D taking all my open tabs with it...
The one thing it DID do right (by design) was that the amount of memory it needed to access to display a given tab was largely isolated per tab, and it didn't need to thrash around the entire process space like Chrome does, meaning 5GB+ of process image could live in swap while the program remained entirely usable. This meant I could open 1000+ tabs even though I only had 320MB RAM; switching to a tab I'd last looked at three weeks ago might take 10-15 seconds (because 100MHz SDRAM) but once the whole tab was swapped in everything would be butter-smooth again. (By "butter-smooth" I mean "20 times faster than Chrome" - on a nearly-20-year-old PC.)
I will warn you that the abstract art that the HTML/CSS parser turns webpages into is an acquired taste.
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Another interesting project in a significantly more developed state is NetSurf, a browser that aims to target HTML5, CSS3 and JS using pure C. The binary is about 3MB right now. The renderer's quality is MUCH higher than Dillo's, but it's perceptibly laggier. This may just be because it's using GTK instead of something like FLTK; I actually suspect firmly kicking GTK out the window will improve responsiveness very significantly, particularly on older hardware.
I have high hopes for this project, but progress is very slow because it's something like a 3-6 man team; Servo has technically already superseded it and is being developed faster too. (Servo has a crash-early policy, instead of trying to be a usable browser, which is why I haven't mentioned it.)
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The most canonical interpretation of what you've asked for that doesn't completely violate the principle of least surprise ("where did all the CSS go?!?! why is the page like THAT? ...wait, no JS!?? nooo") would have to be stock WebKit.
There are sadly very few browsers that integrate canonical WebKit; the GNOME web browser (Midori) apparently does. Thing is, you lose WebRTC and a few other goodies, and you have to lug around a laundry list of "yeah, Safari doesn't do that" (since you're using Safari's engine) but I keep hearing stories of people who switch from Chrome back to Safari on macOS with unilaterally positive noises about their battery life and system responsiveness.
I've been seriously think-tanking how to build a WebKit-based web browser that's actually decent, but at this exact moment I'm keeping a close eye on Firefox. If FF manages to keep the bulk of its extension repository in functioning order and go fully multiprocess, the browser may see a bit of a renaissance, which would be really nice to witness.