Things like the differences between padding and margin, things like negative margins, CSS precedence, !important, box models, floats, clearing floats, vertical centering, and what kinds of layouts can be done with CSS versus which ones simply require JavaScript due to CSS's bizarre deficiencies at even the most trivial things -- none of these things can be learned quickly, and they seem to require many months of experience to actually get right consistently in practice. And then there are the differing philosophies on how to get CSS not to turn into spaghetti in practice, with things like "object-oriented" CSS and tools like LESS/SASS.
And just stupid things like not being able to define "position: relative" on a <td> so that you can absolutely position things within it, the way you can within a <div>. It may not be a browser quirk, it's just a CSS quirk, but I don't really care -- there are TONS of stupid "quirks" in CSS to keep people in their jobs still. Browsers may not suck any more, but CSS still sure does.