A while back, I asked the question of what the XHTML 1.1 doctype is. The answer is found on the W3.org site here. It is this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
They suggest that the xmlns and xml:lang attributes be set for the html tag like this:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
Also, if you are up to it, add the xml declaration to it (<?xml version="1.0" encoding="UTF-8"?> at the top).
I am interested in this because I've been working on redesigning my main web site to make it more colorful, easier to update, and more modern in the code. When it's finished I'll be sure to post everywhere to let you know.