XHTML

XHTML stands for Extensible Hypertext Transfer Markup Language. XHTML is HTML based on XML. It is the also newer version of HTML, and is used to make websites and webpages. There will be no more newer versions of HTML, after HTML 4.01, so XHTML will replace HTML.

XHTML is much more strict then ordinary HTML. In example you have to close all tags, even those without a closing tag (<br> becomes <br />).