Not THAT non-trivial. I built overlapping tabs using images and JavaScript in 1997 - that wasn't more than a few hours' work (couldn't have been - I have the attention span of a door handle).

http://www.wittenburg.co.uk/mobile

Click "Handsets" on the top menu.

Select any phone on the left.

Tabs are shown on the right.

[Edit] Not saying joshuacc's CSS version is trivial. It's not. And it IS impressive!