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!
- Conversion/checking tools are available (eg, tidy)
- XML Editing tools begin to be "XHTML Aware"