Interesting comments from those guys.
I taught myself CSS by reading the CSS 1 spec back in the late 90s (mainly aimed at IE 4 - Netscape 4.6 sucked bad) before there were any books on it. I think the CSS 2 spec was also out by then too, but the browsers didn't support it yet (that took a surprising long time).
I found learning CSS 1 from the spec pretty easy as it wasn't a large spec and was fairly easy to read. The CSS 2 spec was a lot larger (5-10 times?), and a lot more complex with all the new stuff. Learning that took some help from the old Eric Meyer book from O'Reilly with the pair of trout (?) on the front.
The Eric Meyer book helped a lot with understanding fundamentals like how flow worked etc and properly grokking inline vs block etc. There wasn't really any of the tooling (eg Firebug and successors) to help back then.
Of course, I wasn't reading the specs to implement a rendering engine - just to use it.
I stopped doing any CSS about 8 yrs ago just as CSS 3 was starting to happen and never really learnt that. It seems way more powerful/complex/useful these days.
One tip that really helped back then was to try and do what you wanted with the very minimum of selectors/declarations to avoid conflicts, ambiguities and debugging difficulty. So creating lots of scratch empty test pages to explore one specific technique before including it with all your other CSS/HTML.