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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