XHTML, first introduced in 2000, is billed as the successor to HTML. It’s short for Extensible Hypertext Markup Language. XHTML 1.0 is essentially a reworking of HTML 4 in XML – Extensible Markup Language. As such, HTML 4 and XHTML 1.0 are very similar.
XHTML is stricter than regular HTML, as you’ll see in a moment. While this extra strictness requires a bit more effort when creating XHTML pages, it does mean that those pages are very easy for computers to read. HTML, in contrast, is notoriously difficult for browsers to interpret — which is partly why no two browsers seem to display a Web page in the same way!