Get Ready For XHTML
XHTML is not very different from the HTML 4.01 standard.
So, bringing your code up to the 4.01 standard is a good start.
In addition, you should start NOW to write your HTML code in lowercase letters, and NEVER skip closing tags (like </p>).
The Most Important Differences:
- XHTML elements must be properly nested
- XHTML elements must always be closed
- XHTML elements must be in lowercase
- XHTML documents must have one root element
see the full reference at here