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Open the menu in the HTML editor and you'll see an option for "Analyze HTML". While we were in there tweaking the rules to be more helpful, we added a new feature that we didn't have before. It will now warn you if you use a class in your HTML that you haven't defined anywhere in your CSS.
This GIF explains:
js-* and it won't warn you. That's a pretty decent best practice anyway, I'd say.