function BH_DOM_Node_lookupNamespaceURI(node, prefix)
{
    // prefixes bound by definition to specific namespace names
    // this behavior might be non-compliant; for details please see
    // http://lists.w3.org/Archives/Public/www-dom/2005OctDec/0116
    
    if (prefix == 'xml')
        return 'http://www.w3.org/XML/1998/namespace';
        
    if (prefix == 'xmlns')
        return 'http://www.w3.org/2000/xmlns/';

    // more lookup is not possible without a node
    if (!node)
        return null;
        
    // treat empty string as null
    if (prefix == "")
        prefix = null;
        
    if (node.nodeType == 9) // DOCUMENT_NODE
        return arguments.callee(node.documentElement, prefix);
        
    if (node.nodeType == 6) // ENTITY_NODE
        return null;
    if (node.nodeType == 12) // NOTATION_NODE
        return null;
    if (node.nodeType == 10) // DOCUMENT_TYPE_NODE
        return null;
    if (node.nodeType == 11) // DOCUMENT_FRAGMENT_NODE
        return null;
        
    if (node.nodeType == 2) // ATTRIBUTE_NODE
        return arguments.callee(node.ownerElement, prefix);
        
    if (node.nodeType != 1) // ELEMENT_NODE
    {
        for (var cur = node.parentNode; cur; cur = cur.parentNode)
        {
            if (cur.nodeType != 1) // ELEMENT_NODE
                continue;

            // lookupNamespaceURI of ancestor element
            return arguments.callee(cur, prefix);
        }
        
        return null;
    }

    // 
    // for elements
    //
    if (node.namespaceURI != null && node.prefix == prefix)
    {
        // Note: prefix could be "null" in this case we are
        // looking for the default namespace 
        return node.namespaceURI;
    }
    
    var attrs = node.attributes;
    
    // for all attributes... this should filter out invalid namespace
    // declarations, but this isn't 100% certain as yet; also, there
    // are some problems with the relevant specs...
    for (var i = 0; i < attrs.length; ++i)
    {
        var attr = attrs.item(i);
        
        // not a namespace declaration
        if (attr.namespaceURI != 'http://www.w3.org/2000/xmlns/')
            continue;
            
        // non default namespace
        if (attr.prefix == "xmlns" && attr.localName == prefix)
        {
            //
            if (attr.value == "")
                return null;
                
            // if this is true the xml namespace was bound to a
            // different prefix which is not allowed
            if (attr.value == 'http://www.w3.org/XML/1998/namespace')
                return null;

            // if this is true the xmlns namespace was bound to
            // a different prefix which is not allowed
            if (attr.value == 'http://www.w3.org/2000/xmlns/')
                return null;
                
            return attr.value;
        }
        
        if (attr.localName == "xmlns" && prefix == null)
        {
            //
            if (attr.value == "")
                return null;
                
            // assume xmlns='http://www.w3.org/XML/1998/namespace' is allowed
            // http://lists.w3.org/Archives/Public/xml-names-editor/2005Dec/0001
            // should have a response if they are not
            if (attr.value == 'http://www.w3.org/XML/1998/namespace')
                return attr.value;

            // assume xmlns='http://www.w3.org/2000/xmlns/' is allowed, cf above
            if (attr.value == 'http://www.w3.org/2000/xmlns/')
                return attr.value;
                
            return attr.value;
        }
        
        for (var cur = node.parentNode; cur; cur = cur.parentNode)
        {
            if (cur.nodeType != 1) // ELEMENT_NODE
                continue;

            // lookupNamespaceURI of ancestor element
            return arguments.callee(cur, prefix);
        }
    }
    
    return null;
}