A complete plan to reduce vulnerability from untoward influences is more feasible now with agent assisted engineering. You have to be carefully selective about what matters and what doesn't, or the cost and time balloon out of control.
I don't see China antagonistic to the west - they want autonomy, and don't share many of the values, but they're open to a wealthy West spending lots of money in Chinese goods.
Russia, at least its government, is a bit more, especially recently. But anyway, why reinvent the wheel when a battle tested FOSS is there for the taking?
One does not simply "write a browser"...
HTML is entirely the wrong comparison as it's a document format and not a programming language, and "as long as you can write a browser" is not a simple condition. Do we have any browser (even in development) that isn't written with current toolchains? Or a headless javascript runtime perhaps?
But I also don't understand why (without compatibility layers) a browser should be as complex, or remotely as complex, as currently, specially with all the knowledge in hindsight.
I'm not saying this is easy, doable, or whatever, but that was what I meant.