XHTML

XHTML is a term that was historically used to describe HTML documents written to conform with XML syntax rules.

The following example shows an HTML document and corresponding "XHTML" document, and the accompanying HTTP Content-Type headers they should be served with.

HTML document

<!-- Content-Type: text/html -->

<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8" />
    <title>HTML</title>
  </head>
  <body>
    <p>I am a HTML document</p>
  </body>
</html>

XHTML document

<!-- Content-Type: application/xhtml+xml -->

<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">
  <head>
    <title>XHTML</title>
  </head>
  <body>
    <p>I am a XHTML document</p>
  </body>
</html>

In practice, very few "XHTML" documents are served over the web with a Content-Type: application/xhtml+xml header. Instead, even though the documents are written to conform to XML syntax rules, they are served with a Content-Type: text/html header — so browsers parse those documents using HTML parsers rather than XML parsers, which can cause a variety of sometimes-very-surprising problems. The problems are described in more details in the following articles: