Introduction of XHTML

XHTML is a web standard which has been agreed by the W3C. Its a combination of XML and HTML.XML was designed to describe data and HTML was designed to display data.Therefore, by combining HTML and XML, and their strengths, we got a markup language XHTML for writing better formatted web page.

Following are some of XHTML feature:
1. Its a case sensitive.
2. All tags must be in lower case.
3. All tags must use id instead of name attribute.
4. Web page must have DOCTYPE tag.
5. DOCTYPE could be strict, transitional or frameset.
6. All attributes of a tag must be quoted.
7. Opening tag must match with closing tag for container tags.
8. Empty tags like line break < br > must be followed by space and forword slash like - < br />.
9. Attributes must not be shortened. Like in case of checkbox tag write < input type="checkbox" checked="checked" > instead of < input type="checkbox" checked >.

Reference:
http://www.freewebmasterhelp.com/tutorials/xhtml
http://www.w3schools.com/xhtml/