For over a decade, web development has been unthinkable without CSS. Yet the way how CSS is authored is far away from the professionalism we use in the rest of our code. Most developers try to avoid touching the project's stylesheets, as these contain unmaintainable spaghetti code. This can be improved by transferring common practices of software engineering to CSS authoring: With some additions, transcompilers like Sass allow to use the KISS and DRY concepts. Applying object-orientation to CSS will make it SOLID. Architectural design patterns like Separation Of Concerns achieve scalability and reusability. Styleguide-driven development allows the agile/lean principle of delaying decisions to the last responsible moment. Besides showing how to apply these practices to CSS, I will explain why they make a positive impact on maintainability.