Did you read the article? The last example pretty clearly explains why it's a presentation concern.

I have, but I still think that putting stuff like this into CSS is a bad idea. Though it solves the problem mentioned in the article.

Explain why you think it is a bad idea. I'm interested.

That was my gut reaction as well. But then thought about it this way. The programmer can solve with problem with z-index and now the designer can add stuff without interfering with the program.