In 2009, Smashing Magazine wrote up a great article about the 10 ways to improve the accessibility of your XHTML website by making it standards-compliant. A few of the things they touched up on were specifying the correct DOCTYPE, one thing we seems to ignore because most of the time our IDEs generate this for us; defining the namespace and the default language; supplying proper meta tags. They go ahead and list 10 of them.
Again, a great article. I like to brush up on the article every few months so I can save time in the long run.