There’s a lot of messy stuff the HTML syntax supports for historical reasons. https://html.spec.whatwg.org/multipage/parsing.html#an-introduction-to-error-hand

There’s a lot of messy stuff the HTML syntax supports for historical reasons. https://html.spec.whatwg.org/multipage/parsing.html#an-introduction-to-error-handling-and-strange-cases-in-the-parser https://html.spec.whatwg.org/multipage/parsing.html#an-intro... covers a variety of fun ones, like how inline formatting elements basically support overlapping markup, and how malformed nesting can be achieved in a few different ways. It’s a useful section of the spec for understanding these things because it explains what’s going on, with links to the precise parser details.