XHTML, or eXtensible Hypertext Markup Language, is a reformulation of the HTML markup language, based on XML. It combines HTML markup tags with XML rules. This standardization process allows web pages to be displayed in this format on various devices (television, PDA, mobile phone, etc.), in addition to improving content accessibility. The main difference between XHTML and HTML is that the former is valid XML, while the latter has its own syntax. Both have semantic meaning. To verify if a page has been correctly built according to the XHTML standard, there are several validators; the W3C provides an online validator.