XHTML - Data notation

XHTML is a data notation created in 2000.

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24Years Old

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. While HTML, prior to HTML5, was defined as an application of Standard Generalized Markup Language (SGML), a flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. Read more on Wikipedia...

XHTML Wikipedia page

Example from hello-world:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/TR/xhtml1" xml:lang="en" lang="en">
  <head>
    <title>Hello World</title>
    <meta charset="UTF-8"/>
  </head>
  <body>
    <p>Hello World</p>
  </body>
</html>