Mesa

Always declare the content type so the browser knows what character set to use to render the site.
 
I use other meta tags, I believe it's still good practice to and until it's more common not to, I'll continue to.
 
W3C certs, not worth it IMO - clients won't care and they're pretty useless.

jQuery Load Content - nice and easy capable of some powerful and impressive (add animations etc) results.
 
 
Documentation: http://api.jquery.com/category/ajax/
Just about the best demo and tut: http://net.tutsplus.com/tutorials/javascript-ajax/how-to-load-in-and-animate-content-with-jquery/

AJAX perhaps? I'd use jQuery myself, if what you ask is what I imagine.