FWIW C++ can do bound checks aka hardened STL. On the other hand google did riir some of their programs and removed bound checks there for speed, welcome to real world, so absence of bound checks is not a mistake, but an intended performance goal.
And you're saying somebody who is not Google can make a brand new browser engine from scratch in 2026, have it perform good enough to be actually usable in practice, and not be an absolute security nightmare?
Also modern standard compliant (without adblock) browsers don't perform anywhere good.
Modern standard compliant doesn't mean showing ads. It means centering the div when you have tables mixed with floating mixed with grid mixed with flexbox, and all sizes are a combination of px, em, % and vw. While rendering an animated SVG. While rendering the CSS animation of that animated SVG. While recording audio. While processing that audio in a background worker. While another tab synchronizes with the current tab using local storage. And so on and so forth. There's thousands of features you need to implements and millions of possible interactions between them. And if you get any of these wrong, some website will be broken. Ignore enough features, leave enough interactions untested, and a huge chunk of the internet will be some degree of broken.