Once upon a time it was considered a great strategy for browsers to make the best effort to display a page, even if the html code had errors. Just try to guess the author's intention as well as you can, and go ahead. Errors are bad. Users don't want errors. I thought we learned from that experience. Apparently not.
If early browsers displayed an error the first time it came across an tag, instead of just best-effort rendering the page, then every new browser feature would have been smothered in the cradle.