OOCSS

Atomic CSS: is the approach to CSS architecture that favors small, single-purpose classes with names based on visual function.

BEM: Block Element Modifier is a methodology that helps you to create reusable components and code sharing in front-end development.

SMACSS: I’ve been analyzing my process (and the process of those around me) and figuring out how best to structure code for projects on a larger scale. What I've found is a process that works equally well for sites small and large.