<script type="text/javascript">
MyModule = function(element)
{
     MyModule.initializeBase(this, [element]);
}

MyModule.prototype =
{
    initialize : function()
    {
        MyModule.callBaseMethod(this, 'initialize');
        var selfPointer = this;
        this.get_editor().add_selectionChange(function (){ selfPointer.doSomething(); });
        this.doSomething();
    },

    //A method that does the actual work - it is usually attached to the "selection changed" editor event
    doSomething : function()
    {
        var span = document.createElement ("SPAN");
        span.innerHTML = this.get_editor().get_html();

        var imageCount = span.getElementsByTagName("IMG").length;
        var tableCount = span.getElementsByTagName("TABLE").length;

        var element = this.get_element();
        element.innerHTML = "<b>CUSTOM MODULE: Images: " + imageCount + " Tables: " + tableCount + "</b>";
        element.style.border = "1px solid red";
        element.style.backgroundColor = "yellow";
        element.style.color = "red";
    }
};

MyModule.registerClass('MyModule', Telerik.Web.UI.Editor.Modules.ModuleBase);
</script>

Another option is to override the methods and functions of the built-in Properties inspector module as shown in this forum thread: Edit html div attributes in design mode.