Mine

function quirks_widget (window) {
    this.name = "quirks-widget";
    text_widget.call (this, window);
    this.add_hook ("content_buffer_location_change_hook", method_caller (this, this.clear_flag))
    this.add_hook ("buffer_dom_content_loaded_hook", method_caller (this, this.set_flag));
    this.add_hook ("select_buffer_hook", method_caller (this, this.update));
}
quirks_widget.prototype.__proto__ = text_widget.prototype;

quirks_widget.prototype.update = function () {
    var buffer = this.window.buffers.current;
    var str = 'foo';
    dumpln ("update");
    if (buffer.quirks_flag == true) {
        if (buffer.top_document.compatMode == 'BackCompat')
            str = 'quirks';
        else
            str = 'standards';
    }      
    this.view.text = str;
}

quirks_widget.prototype.clear_flag = function () {
    dumpln ("clear_flag")
    this.window.buffers.current.quirks_flag = false;
    this.update ();
}

quirks_widget.prototype.set_flag = function () {
    dumpln ("set_flag");
    this.window.buffers.current.quirks_flag = true;
    this.update ();
}

add_hook ("mode_line_hook", mode_line_adder (quirks_widget));