A good manual / tutorial / reference to learn CSS

Hi support team,

In the past year I learned a lot about HTML5 and CSS. I'm continuously learning, but sometimes I wish I knwew certain things beforehand.
For example, in topic #1520738 David came with a good suggestion to specify the Top padding to make sure the hero image was displayed correctly. I strugled a long team to solve this on our site but never managed to find a solution.

Can you suggest me good learning or reference material so I could improve my knowledge about CSS and more of these topics?

I know it's probably a difficult question, but any suggestion would help.

Thanks in advance,
Henk.