Overlapping tabbed navigation in CSS have always been a nuisance with me often running into cross-browser compatibility issues or bloated code when developing them. There are some other examples floating around the internet, though it seems there aren't any really practical cross-browser compatible, accessible, expandable (menu item length and menu item quantity) and using minimal CSS and XHTML.
I have created what I believe to be a working prototype, though I am no accessibility expert and am open to opinions and criticism. You can see four of the tabbed navigation examples below:
These navigation tabs are completely customisable and tested in IE 5.5, 6, 7, Firefox 2.0.0.13, Opera 9.25, Safari 3.0.4 (All on Windows). If anyone finds any incompatibilities, please don't hesitate to leave a comment and I'll try to fix it straight away.
The CSS also uses the box model hack to support IE5. If you wish the CSS to validate, then you can sacrifice the IE5 support to accomodate it.
The example above uses an CSS image preloader technique. This is not essential though it is highly recommended as otherwise the menu rollovers will suffer some lag. More information on this technique can be found here.
You can use these menus for free; the source is available below:
7