HTML allows for creating hypermedia documents, which plain text does not. In small, reasonable amounts, it provides functionality not easily reproducable with a plain-text interface while still being reasonable.
Stacking on lots and lots of other stuff, though, is when it becomes silly.
True, true. Hyperlinks are pretty big. Defining links with a new language is certainly one way to do it. But look here on HN, what do I do? I write plaintext. I certainly don't write <a href>. I write like this [1], and the hyperlink becomes an implementation detail. An HTML implementation detail, haha, sure, but nowadays it's not hard for hyperlinks to be emulated. Anyhow I honestly do think html5 is a great language. (but so is text! :)
Stacking on lots and lots of other stuff, though, is when it becomes silly.