[nicoburns]

I believe before html5 they weren't required, but what the parsed DOM tree looked like wasn't clearly defined if they weren't, and in practice different browsers did it differently meaning you could very easily end up with it rendering differently in different browsers. html5 fixed that.

[anjbe]

This is not correct. What you’re describing sounds like how HTML5 standardized parsing of invalid HTML, but that is not the same thing as implicit closing tags, which have have always been valid, correct HTML producing an unambiguous, clearly defined result.

[niconii]

Oh, for sure, actually parsing HTML was awful before HTML5. The spec sometimes diverged from how browsers actually interpreted HTML, and error correction basically boiled down to browsers trying to reverse-engineer each other to figure out how they handled broken HTML. HTML5 was a godsend for actually standardizing all of that.