<p>I have seen css navigation designs done mostly through <code>ul</code> and <code>li</code> lists for the tab structure. What is the reason behind this?. What is the fall-back for such sites if css is somehow disabled?</p>

What's the reason behind using UL Li lists to draw navigation bars?

I have seen css navigation designs done mostly through ul and li lists for the tab structure. What is the reason behind this?. What is the fall-back for such sites if css is somehow disabled?

Semantic markup has various reasons, for example:

screen readers can access the site more easily
google bot can crawl the site more easily

This link gives more detail on the topic: Semantic HTML. Besides, it is HIGHLY unlikely someone will have CSS disabled. In my opinion there is absolutely no need to provide fallback for that case, the navigation would still be accessible anyway.