So long as you don’t read layout information in between, you can already batch your modifications just fine. All it takes is care in how you structure and implement things, and you’re fine. And before you object, your proposed solution would require almost as much care, have more hazards to trip over, and require opting in in a way that few would—and those that would, already know how to be careful. I suspect it would also increase complexity and possibly memory usage in the browser.

If you were designing something from scratch, such approaches would be worthwhile considering, but I think that boat has sailed, and the architecture would fight against you.

Then again, I believe it was generally accepted that web browsers were stuck with UTF-16, until Simon introduced WTF-8 for Servo.