I used LESS a lot. These days I decided to try SASS and to be honest it's a better choice for CSS preprocessor. It just gives me more functionalities and better control on my code. There are few instruments for architecting your CSS logic - @mixin, @extend and placeholders. However there are differences between them and specific use cases.
I ❤️ markdown. I like its simplicity and minimalistic API which is good enough to cover most of needed HTML markup. At least for textual content. At work we have a small node based microservice that delivers data from Contentful in exactly markdown format. It's all working well but we started having cases where part of the text is in Contentful and the other part is in that microservice. For example when we have a button with a catchy design. We want to content manage the label of the button but the actual markup to be on our side.