We have entered the Age of Components. Modern frameworks use components as their foundation, allowing you to break your app into simple single-purpose parts that can be composed together to solve more complex needs.

Unfortunately, CSS tends to solve problems from the top down, starting with more general rules to more specific rules. These contradictory methodologies can lead to some frustrating decisions as you try to compose your app using lower-level components.

Many tools have been created to help us manage our CSS. Tools like SASS, Less, CSS-Modules, Post-CSS, and CSS-in-JS all solve the problem of maintaining CSS style sheets, but they all fall short in the one problem that tooling can never solve: Which elements should own which style properties? This answer to this question is key to making composition work, especially where web layout is concerned. By the end of this talk, you will know the answer to this question and start composing layouts in your web app.