Yes. The same (x)html and CSS will work fine on mobiles. You can offer up slightly different CSS to mobiles if you want that are optimized for mobiles, but it's not essential. It's just an optional enhancement.
EDIT: of course, different mobiles have different support for CSS. I'm thinking of smartphones like iPhone and Android devices, but any mobile with web connectivity will be happy with html and some CSS. For really simple mobiles, the key is to keep styles very simple--just a bit of coloring, basically. Fancy layouts will struggle to display well.
One thing you can do is view a site in Opera and choose mobile view to get a sense of what the mobile view might look like. There are also some online testing tools that purport to give you some idea of how various mobiles will render the site, such as http://www.springbox.com/mobilizer/