XML Technologies: a Key for Interoperability - slide "Minimal XHTML 1.0 Document"

The header has to be XML-like:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/strict.dtd">
<html xmlns="http://www.w3.org/TR/xhtml1/strict" xml:lang="en" >
  <head>
    <title>A Small Document</title>
  </head>
  <body>
    <p>A simple page!</p>
 </body>
</html>

95% of the browsers in use can serve XHTML!