[Frooxius]

It's not about enclosing <html> ... </html> tags, it's about tags in general. You can post a HTML code snippet that's not contained within these tags (so it's not a full page) and it's still a full HTML code, but... HTML is a markup language, it contains tags, it doesn't allow you to describe any logic itself, just a way to mark and describe content.
The code you presented is JavaScript and it uses HTML DOM to interact with the HTML elements, but the language is JavaScript which you can tell by the syntax and the fact that it's the most common language for website scripting.