Sure you might know CSS inside out, but I'd be willing to bet some people here could learn something useful out of it. The rest of us that already are familiar with it get a quick refresher in under 5 minutes.

Positioning and the box model are at the core of understanding CSS layouts. Sure it's not the next cool thing and it certainly isn't advanced, but I'd much rather browse a Web where people understand layouts properly, than one filled with a lot of the trendy, over-complicated crap that fails at the basics.

We have to be careful with that basic = not challenging = not geek-worthy attitude. A beginner reading HN comments will most likely emulate it and miss out on the important stuff.

This looks like the best example I have so far to pass along to people asking me about css, which happens quite often.

That aside CSS is messy and complicated, lots of programmers dont understand the basics, and even experienced developers can easily miss a few very useful tips.

One of the most helpful things I ever did with respect to CSS was read the apress book on html/css patterns (just ignore the parts that use javascript).

http://books.google.com/books?id=xq9IOomk1tUC

(I never realized that standard CSS actually operates with 6 box models!)

If the reader does know CSS, then they're probably asked "can you make this for me?" by everyone and their extended family, and this is concise and has immediate, clear examples to make it newbie-friendly. I wouldn't dare throw a big CSS book at a non-programmer new to CSS.