I tried adding a a new email body with just a button in and nothing else to see how it would render but I also got the same result. It appears fine in the Preview window and when lauched from there it is still looking fine but when I view the source code I noticed that the closing HTML tag is duplicated?
<html><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<input type="button" name="button" id="button" value="Button" />