I learned CSS over the years by gradually solving problems I encountered building apps. Compare this to people learning CSS now as evidenced by the #100DaysOfCode tag on Twitter. The learning technique is comprised primarily of using gradient-heavy, absolutely positioned HTML elements to create a photo-realistic, 3D rendering of objects.
The results are pretty amazing, but I have my doubts about whether these skills are easily translatable for building an interactive, responsive UI. Some examples:
https://twitter.com/bauervadim/status/1282264611912327169
https://twitter.com/mercyoncode/status/1282449080132804609
https://twitter.com/ellie_html/status/1276177277315932161
https://twitter.com/thecoffeejesus/status/128204582508278169...