There are lots of tools for finding unused CSS rules in stylesheets, such as [Chrome Audits](https://developer.chrome.com/devtools#audits) and the [Dust-Me Selectors](https://addons.mozilla.org/nl/firefox/addon/dust-me-selectors/) add-on for Firefox (unfortunately not compatible with Firefox Quantum).

But what about the other way around?

How do I find classes that are in my HTML, that do not exist in my stylesheets?