There's a ton of information in the CSS already - anything that uses color to denote meaning, or the layout of a component, or the order of items in a flex list, and so on. That's all data in a page's information hierarchy. Plus, the content attribute of the ::before and ::after pseudoelements literally exists for injecting content in to a page using CSS. Why not use it?
Imho, labels in the CSS are pretty tricky for i18n. The colors doesn't (generally), need traductions, they have the same meaning whatever the language.
Data attributes in another hand, despite of being a bit redondant (in fact, it's not that insane in a loop ^^'), can be easily translated in HTML.
