public function start_html() {
        global $OUTPUT;
        $this->id = 'core_progress_display_if_slow' . self::$nextid;
        self::$nextid++;

        // Containing div includes a CSS class so that it can be themed if required,
        // and an id so it can be automatically hidden at end.
        echo \html_writer::start_div('core_progress_display_if_slow',
                array('id' => $this->id));

        // Display optional heading.
        if ($this->heading !== '') {
            echo $OUTPUT->heading($this->heading, 3);
        }

        // Use base class to display progress bar.
        parent::start_html();
    }