HTML is a programming language. There, I said it.
It's a programming language because it lets people tell the computer to do cool stuff. In this case, show all kinds of things on screen. That's not much, you say? So what. It's fun. Meaningful. Often even useful.
So, you can't use HTML to perform computation. Big deal. It's been a long time since computers were all, or even mainly, about computation. Deal with it.
HTML5
I once described HTML5 as an admission by manufacturers that major browsers never truly implemented the previous 4.0 standard. That's a bad thing; as a friend pointed out, web standards are now dictated by Google, and we're even worse off (in this regard) than two decades ago before Firefox came out. But at least we're more honest now about how the web actually works in practice.
Besides, you can use it for example to add graphics right into a web page via SVG.