Standardize that HTML Documents return HTML

Long ago, before HTML5, I preferred XHTML 1.0 over HTML 4.X and so I
had the HTML Document set to output XML in it's __toString. At some
point between then and now I changed to using HTML as most of my
templates use HTML 5 now. All of this is to say that I have removed
some vestigial items and standardized on HTML output for HTML Document.