Prova questo ad esempio. Fa uso dell'innerHTML che non e' DOM ma e' ben supportato

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "DTD/xhtml1-transitional.dtd">
<html>
<head>
    <script type="text/javascript">
        function Oggetto(width, height) {
            this.container = document.createElement('div');
            this.div = document.createElement('div');
            this.div.style.width = width + "px";
            this.div.style.height = height + "px";
            this.div.style.backgroundColor = "#cccccc";
            this.container.appendChild(this.div);
            
            
        }
        
        Oggetto.prototype.toString = function()
        {
            return this.container.innerHTML;
        }
    
    </script>
</head>
<body>
    <div id="qui">
        <script>
        document.writeln(new Oggetto(100,100));
        </script>
    </div>

            
</body>
</html>