protected function echoPageLeader(): void
  {
    echo '<!DOCTYPE html>';
    echo Html::generateTag('html',
                           ['xmlns'    => 'http://www.w3.org/1999/xhtml',
                            'xml:lang' => Abc::$babel->getCode(),
                            'lang'     => Abc::$babel->getCode()]);
    echo '<head>';

    // Echo the meta tags.
    Abc::$assets->echoMetaTags();

    // Echo the title of the XHTML document.
    Abc::$assets->echoPageTitle();

    // Echo style sheets (if any).
    Abc::$assets->echoCascadingStyleSheets();

    echo '</head><body>';
  }