I made a video (8:41 minutes) that explains four of the most important rules for properly coding XHTML. If you follow these rules, you'll avoid about 90 - 95% of the validation errors that student often make at the beginning of the quarter.
These are the rules (but watch the video to get a better understanding of them):
- Documents must be well formed.
- All element and attribute names must be written in lower case letters.
- Attribute values must always be quoted.
- Elements must be closed including empty elements.