I have the Tshirt for the same mistake and now program using the following which shows shows errors and warnings:
1. Use this in your header
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
2. Use Firefox Addon:
https://addons.mozilla.org/en-US/firefox/addon/html-validator/
Notice: bottom right hand corner icon “1 error / 0 warnings”