The idea of the post is to show that dichotomy of imperative/declarative is false. It is rather a spectrum.
Different parts of CSS are on different levels of the spectrum. If we would consider position and flow, CSS hardly classifies as declarative, because you can't specify "declaratively" even simple idea of centering something relative to something else. But specifying colors is pretty "declarative".
One more example of implementation detail, which CSS makes me think about is measurement units
See the video from decostructconf (link in the post). There is an explanation, how this task could be solved in a more "declarative" way
I see now. From the way you put it, I definitely agree that it is indeed a spectrum.