HTML is expressed as SGML and XHTML is expressed in XML. Creating XHTML is connected with more restrictions in the form of markup.
Avoid using the <video> or <audio> tags in XHTML 1.0 Transitional, as those are not an element of that specification.
Convert from HTML to XHTML
- Add an XHTML <!DOCTYPE> to the first line of every page
- Add a xmlns attribute to the HTML element of every page
- Change all element names to lowercase
- Close all empty elements
- Change all attribute names to lowercase
- Quote all attribute values