A while back I saw the Trello CSS Guide swim around my Twitter timeline. I've been using that style of writing CSS ever since, I've even started to introduce it ever so slightly in the Bloglovin CSS (although converting a two year old project is not something you do in one go). I've found the guide to result in very maintainable CSS code. It's even possible to make changes without being scared of breaking stuff, because it's all so neatly namespaced.
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