1. HTML documents are made of elements that have three different components- element tags , start tag end tag; element attributes are given within tags. HTML element lies between and including tags.
XHTML documents has only one root element. All elements and variables must be in lower case, and values assigned must be surrounded by quotation marks, closed and nested. This is a mandatory requirement in XHTML but in HTML it is optional.
XHTML is a family of XML languages which is the mirror version of HTML. It does not allow omission of any tags. It requires that there should be an end tag to every start tag and all tags must be closed in the right order. For example, <br> is a valid HTML tag, but in XHTML it should be written as <br />.
3. HTML is application of Standard Generalized Markup Language(SGML). It is the standard text formatting language used for creating and displaying pages on the Web. Whereas XML is application of Extensible Markup Language(XML).XHTML is easier to teach and to learn. XHTML is XSL ready.
4. HTML is a static Web Page whereas XHTML is dynamic Web Page.
5. Developer can do changes in HTML tags whereas in XHTML when user needs a new tag then user can define it.
6. HTML is about displaying information whereas XHTML is about describing the information.