From http://www.w3schools.com (Copyright Refsnes Data)

XHTML - Why?

XHTML is a combination of HTML and XML (EXtensible Markup Language).

XHTML consists of all the elements in HTML 4.01 combined with the syntax of XML.

Why XHTML?

We have reached a point where many pages on the WWW contain "bad" HTML.

The following HTML code will work fine if you view it in a browser, even if it does not follow the HTML rules:

<html>
<head>
<title>This is bad HTML</title>
<body>
<h1>Bad HTML
</body>

XML is a markup language where everything has to be marked up correctly, which results in "well-formed" documents.

XML was designed to describe data and HTML was designed to display data.

Today's market consists of different browser technologies, some browsers run Internet on computers, and some browsers run Internet on mobile phones or other small devices. The last-mentioned do not have the resources or power to interpret a "bad" markup language.

Therefore - by combining HTML and XML, and their strengths, we got a markup language that is useful now and in the future - XHTML.