Earlier today I discovered HTMLHint, a linter for HTML documented. Based on how easy it is to write linting extensions for Brackets, I downloaded the library and wrote a wrapper in approximately 10 minutes. Here's a screen shot of in action.

The linter itself is still a work in progress and doesn't seem to validate tag names themselves (I was able to use a poo tag), but seems pretty handy in general.

You can install this now via the extension manager or grab the bits from GitHub: https://github.com/cfjedimaster/brackets-htmlhint

As a quick aside - I'm ashamed to admit that I have not always used linters. I had a JavaScript linter for Brackets (JSHint) but I didn't use it all the time. Now that the linting API can run linters automatically and pop open a set of issues I'm finding myself relying on them more and more. As an example, it is great for finding bugs that I'd have missed until I actually ran my code.