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:

It doesn't factor in JavaScript, so feel free to ignore its warnings if that's why those class names are there. But to avoid the warning entirely for JavaScript-related classes, start the class with js-* and it won't warn you. That's a pretty decent best practice anyway, I'd say.