The combo between HTML and XML.

Basically the big differences between HTML and XHTML are:

• Tags must be closed properly!
• uppercase letters are not to be used in tags
• Elements must be properly nested i.e. <tag 1> <tag 2>… </tag2> </tag1> note that because tag 2 is nested in tag 1 it must be closed before you close tag 1.
• Empty elements like line breaks have to be closed all in one go - i.e. <br />
• attributes like width color etc. must be “quoted

The difference between strict and transitional? The type of XHTML you use should only be transitional when you want to use the presentational features of HTML