CSS/XHTML and included files?!

mmm probably a stupid Q but im confusing myself

http://www.twenty-eleven.com.fidowebsite/index.html
this is my first non tables standards compliant site and seems to be going OK!?

but, i want to have the nav in a seperate file so can be updated sitewide - have previously used php includes but cant do that here or it wont be valid markup, right??!!

been learning my CSS by mistakes for a few hrs now so forgive me if im just not seeing a simple solution here!!!

cheers all