There are more current browsers that support HTML 5 than support XHTML. If you want to use XHTML then you no longer support some of the browsers that support (or can be made to support) HTML 5.
It shouldn’t be much longer though before XHTML 1 or XHTML 5 can be used though (IE8 is the last commonly used browser that doesn’t support it).
Haha. Sorry about that! You can remove it. That’s created when I hover over text parts of an image. It’s from a pretty cool Chrome Extension called Project Naptha that allows you to copy text from within images. I found it via Versionining if I’m not mistaken.