HTML5 is a markup language used to structure and present content for the World Wide Web and a fifth revision of the HTML standard. It is developed by World Wide Web Consortium and Web Hypertext Application Technology Working Group (WHATWG).
Features of HTML5:
XHTML5 is the XML serialization of HTML5. XHTML5 requires XML's strict, well-formed syntax. In XHTML5, the HTML5 doctype html is optional and may simply be omitted.
HTML5 is designed so that old browsers can safely ignore new HTML5 constructs. Although HTML5 now defines a consistent behavior for "tag soup" documents, those documents are not regarded as conforming to the HTML5 standard.