This kind of critique is applicable to all sorts of specialized writing. The flipside is that an article of this caliber (And it is indeed a very useful and well written set of guidelines for the intended audience) is not truly relevant to the layperson or an absolute beginner -- he lays out common problems and frustrations that people who've ever tried to layout a moderately complex website would have encountered, and presents some good solutions for them. If you haven't worked in the field and encountered these sort of problems, then an article like this is simply not relevant.

It is important for the field to be welcoming and open to curious individuals who want to learn, but there is also a very real need to have literature to communicate ideas that industry professionals encounter, and that necessarily implies some level of baseline knowledge. In other words, there are already plenty of resources to begin learning CSS, and not every article has to assume absolutely no proficiency in it. Every discipline of course has plenty of specialized writing. If you pick up a Quantum Mechanics paper, for instance, you'll find that it doesn't spend much time building up mathematical fundamentals like tensor algebra or topology even though it may well be required knowledge for someone to understand the paper.