XHTML
XHTML stands for Extensible HyperText Markup Language.
It is a stricter, XML-based version of HTML: every element must be closed (even empty ones like <br />), tag names are lowercase, attribute values are always quoted, and elements must be properly nested. When a page is served as XML, the browser stops at the first error instead of guessing what was meant.
XHTML 1.0 was published in 2000. HTML5 has since replaced it for most websites, but XHTML is still used in EPUB e-books and in systems that process web pages as XML.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>An XHTML document</title>
</head>
<body>
<h1>My first heading</h1>
<p>Empty elements are closed with a slash: <br /></p>
<p><img src="logo.png" alt="Company logo" /></p>
<h2>Second heading</h2>
<p>Tags are lowercase and attribute values are always quoted.</p>
</body>
</html>