Q:

Changing  to  causes my wrapper to not extend

I just checked my personal landing page on the W3C Validator to make sure it was correct HTML5, and found out I needed to have <! DOCTYPE html> instead of <! DOCTYPE> to pass.  
However the background image won't extend to the bottom.  
I've tried min-heights for the wrapper, and clears, but nothing seems to work.  
Normal: http://gbhall.com/
<! DOCTYPE html>: http://gbhall.com/index2.php
Edit: You'll no longer be able to see the error as I've updated it to the solution provided below.  

A:

You need to give your body and html 100% height also:
html, body { height: 100%; }

