Any recommendation for learning intermediate to advance CSS

Im an experienced backed developer having basic knowledge of HTML & CSS, now I want to improve my skills and looking for your suggestions.

I have gone through many HTML & CSS courses out there, but most of such courses are heavily focused on the basics or provide very little training on the advance.

I learned my HTML & CSS on pirple.com the course is very good and the assignment based learning really help, since the key for programming is keep programming.