XHTML is similar to HTML but much stricter. XHTML defines the following strict requirements: • Write all code in lowercase letters • Encase all attribute values in double quotes • Each tag must have an end specified. This is normally done with an ending tag, but a special case allows for non-content tags.

In HTML5 the MIME-type determines if the HTLM is strict XML or standard HTML. A document served as "text/html" must be a HTML document, and a document served with an XML MIME type such as "application/xhtml+xml" must conform to the requirements of HML.