Pick a few CSS frameworks, say Bootstrap, MDL web components, and Bulma. Choose 5 components from each that you want to learn how to build. Look at the source

Pick a few CSS frameworks, say Bootstrap, MDL web components, and Bulma. Choose 5 components from each that you want to learn how to build. Look at the source code with the goal of understanding how they're accomplishing what they're accomplishing. Try to build a project from scratch, without a framework.