From the folks who invented CSS and HTML, this index lists both HTML and XHTML tags. The items with a D in the Depr. column are deprecated, meaning they're being phased out. They don't exist in strict XHTML 1.1 and won't exist in future specifications either.
Here you'll find a brief description of how the W3Schools Web site upgraded from traditional HTML to XHTML. This is actually the fifth in their brief Introduction to XHTML tutorial.
For quick one-page printed lists of all the CSS properties and XHTML tags, consider printing the CSS Cheat Sheet (V2) and HTML Cheat Sheet pages from this site. You'll need Adobe Acrobat Reader for the pdf versions. You can download Acrobat Reader for free from www.adobe.com.
Here's a page that talks about style sheets and the tag. It goes beyond that into topics you don't really need to know for XHTML. But you might want to add it to your favorites collection for future reference.
Looking for just the right color for your page or an element? Click this link for thousands of color codes. This is one site you'll surely want to bookmark or add to your favorites.
Here's a great resource for special characters. If you don't already have such a resource in your browser's bookmarks or favorites, consider adding this one.