ok this is that i know
-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.
basically, XHTML is HTML (all the html tags are found in XHTML) that follows the rules of XML (because it is a family of XML).
plz dont forget to vote positively on my answer ^^!
good look :)