XHTML is an updated version of HTML that addresses browser compatibility issues. It was developed to accommodate the rapid growth of technology and different browser capabilities. While the coding may change in XHTML, files still use the .html file extension. XHTML is based on XML, which is a more complex language than HTML. Proper XHTML requires elements and attributes to be in lowercase, tags to be closed correctly, empty elements to use a closing slash, proper nesting of tags, and one root html element.