Early web browsers supported only a very simple version of HTML. The rapid development of proprietary web browsers led to the development of non-standard dialects of HTML, leading to problems with browsers. Modern web browsers support HTML and XHTML, which should be rendered in the same way by all browsers.
Extensible Hypertext Markup Language (XHTML) is part of the family of XML markup languages. It mirrors or extends versions of the widely used Hypertext Markup Language (HTML), the language in which web pages are formulated.
HTML is not a programming language; it is a markup language, and is used to tell your browser how to display the webpages you visit. It can be as complicated or as simple as the web designer wishes it to be. HTML consists of a series of elements, which you use to enclose, or wrap, different parts of the content to make it appear a certain way, or act a certain way. The enclosing tags can make a word or an image a hyperlink to somewhere else, can italicize words, and can make font bigger or smaller, and so on.