The DocType header is actually defined by default in the default.aspx page which serves all the other elements like your description, key words (which you can set in dnn per page)
By having this flexibility you have backward compatibility in the look and feel of the skins and independence on multiportal installations, so that if you've got a bunch of skins that require XHTML & Strict header types, you can use them without affecting any other skins in your site or subsequent portals.
This is basically a move forward to providing more compliancy and backward compatibility - allowing the designer more choices.
Does this help give more outline.
Nina Meiers