After 10+ years of making websites, you'd think I already know CSS. But I just signed up for Josh W. Comeau's #CSSforJS course and I'm sure I will learn a lot!
You should check it out, and you can follow my experience taking the course in my website: https://noeldemartin.com/tasks/learning-css
This is an instance-of-one managed by Noel De Martin.
Also, thanks to @moodle for the purchase! I got it using EduCash, a training allowance they give to employees.