Comment by dragonwriter

> What I never understood was why, for HTML specifically, syntax errors are such a fundamental unsolvable problem that it's essential that browsers accept bad content.

Because HTML is a content language, and at any given time the main purpose of the main engines using it will be to access a large array of content that is older than the newest revision of the language, and anything that creates significant incompatibilities or forces completely rewrites of large bodies of work to incorporate new features in a standard is simply not going to be implemented as specified by the major implementers (it will either not be implemented at all, or will be modified), because it is hostile what the implementations are used for.