CSS in 2016
So this is roughly how I handle CSS these days. As much as I'd like to use CSSModules for everything, I work on a lot of different projects, for a lot of different clients. They can't all be a SPA.
Some of this seems blindingly obvious. But until I stop cleaning up messy, repetitive CSS, I figure it all merits being said.
Create a Baseline
I use one file to style HTML. This creates a baseline for the body, defines my box model, sets global typography rules, etc. If I need to style an HTML element, I style it globally. Otherwise, I give an element a class, and only style that class.