<?php
namespace DOM {
    abstract class Document extends DOM\Node implements DOM\ParentNode {
        /* all properties and methods that are common and sensible for both XML
         * & HTML documents */
    }

    final class XMLDocument extends Document {
        /* XML specific properties and methods */
    }

    final class HTMLDocument extends Document {
        /* HTML specific properties and methods */
    }
}

class DOMDocument extends DOM\Document {
    /* Keep methods, properties, and constructor the same as they are now */
}
?>