Developers often need XHTML-compliant HTML markup when they fetch DOM elements' innerHTML. Since the W3C hasn't standardized on this property (for whatever reason) the browsers have been inconsistent in their approaches to innerHTML. Firefox doesn't put the trailing slash in <br> tags, for instance, while Internet Explorer shows tags in all-caps and strips the quotation marks from single-word attributes.
The original intent was to add innerXHTML() and outerXHTML() to the prototype of Element, but Internet Explorer doesn't expose a prototype for DOM elements. The innerXHTML() and outerXHTML() functions are implemented anyway, and are added to the Element prototype for browsers that support Element prototypes. The function xhtml() for innerXHTML() equivalence is also added to jQuery.