XHTML is a markup language for the router and the development of HTML that has the ability more or less similar to HTML, but with a more strict syntax rules. HTML is an application of SGML (Standard Generalized Markup Language) that is very flexible, whereas XHTML is an application of XML, SGML derivative that is more limited.
Because XHTML must have order-form (follow the proper syntax), XHTML documents can be automatically processed using standard XML processing - not like HTML which requires processing that is quite difficult and complex. XHTML can be thought of as a combination between HTML and XML as it is a re-formulation of HTML in XML form. XHTML 1.0 has been the recommendation of the W3C or World Wide Web Consortium on January 26, 2000.