> It would greatly simplify browser code and HTML spec.
I doubt it would make a dent - e.g. in the "skipping <head>" case, you'd be replacing the error recovery mechanism of "jump to the next insertion mode" with "display an error", but a) you'd still need the code path to handle it, b) now you're in the business of producing good error messages which is notoriously difficult.
Something that would actually make the parser a lot simpler is removing document.write, which has been obsolete ever since the introduction of the DOM and whose main remaining real world use-case seems to be ad delivery. (If it's not clear why this would help, consider that document.write can write scripts that call document.write, etc.)