When I first started mobile first design I used a pretty rigid template for my LESS files. All my designs were split across 5 or so LESS files named: mobile.less, tablet.less, desktop.less, etc. This seemed like a good idea at the time, but I quickly created a mess of impossible to maintain rubbish. I realized, I needed to embrace modules. Here is how I tackled the challenge of creating a maintainable UI library that was mobile first, responsive, and modular.