I'm saying the opposite - security is so much of a problem that even the most popular browser in the world can't get it 100% right after 20 years of trying to get it right. Subtract popularity, subtract 20 years, subtract FAANG - do you see how abysmal the security would be in this hypothetical brand new browser engine? There would be a critical 0-day every single day for multiple years straight. People would keep getting their bank accounts hacked on regular basis. And yes, Rust isn't going to help.

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.