Do you make your websites with valid XHTML or cant you be bothered. I think you need to use XHTML because the code is neater and all the tags are closed. XHTML is the revamped HTML, now as HTML is old. Most website use XHTML because of its W3C reccomendation.
I recommend that you have your sites in XHTML and validate them using the W3C validator which can be found at: http://validator.w3.org/.
Does you use XHTML? And do you think that it is nessacary?