XHTML has finally made it as a recommendation, paving the way for modular XHTML to build a future for mobile and TV based browsing. For the uninitiated, these are the differences between HTML and XHTML:

Documents must be well-formed

No overlapping tags and all tags must close properly

Element and attribute names must be in lower case

This is because XML is case-sensitive, and someone had to make a decision one way or another

For non-empty elements, end tags are required

No more leaving off </td> or </p>

Attribute values must always be quoted

This was always good practice, I guess only the lazy web designer leaves off the quotes these days (unless you're trying to save 90 bytes per page)

Attribute Minimization

This is quite a big change - it means that things like 'checked' or 'nowrap' are now invalid

Empty Elements

You need to terminate empty elements with a / like this: <br />. This applies to all empty elements such as IMG, META, HR or BR

Whitespace handling in attribute values

All multiple spaces in attributes now reduce to a single space

Script and Style elements

Ugly, ugly , ugly change. Because of the way they've declared <script> and <style> you have to escape your code in <![CDATA[ . . . ]]> constructs.

SGML exclusions

This just means that they can't use a DTD to describe some of the restrictions they are placing on XHTML such as no nesting <a> elements.

The elements with 'id' and 'name' attributes

You can't use a 'name' attribute as an identifier any more, you should be using 'id'. This affects named anchors and Netscape JavaScript that does image rollovers