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I finally completed Josh W Comeau's course, and I have to say it's the most comprehensive course I've ever seen. It should be required material for anyone working with CSS (I'm not kidding).
And to top it off, it's super fun!
100% recommend: https://css-for-js.dev/
If you're interested in the full recount of my experience going through the course, you can read it in my website: https://noeldemartin.com/tasks/learning-css