Allow HTML/HEAD/BODY elements to be editable

[msporny]

From Brian Sletten:

About adding other namespaces... obviously you can do it at the element level, it would be nice to have some way of doing so for the document. Perhaps make the HTML block contentEditable and parse that?

From Manu Sporny:

I was thinking that people would just put in their own header block in the editing window and I'd dynamically remove the "static content" if I detect an HTML or HEAD or BODY tag. That way, people can just cut/paste their documents into the textarea and the web app does the right/smart thing.