Yes. It works now. I was missing the init() and I thought that render() actually outputs the html and not simply returns it.
The following can be inserted in a TD tag in a html table:
$app = new \atk4\ui\App('Home Hub', ['icon'=>'user']);
$layout = $app->layout;
$table = new \atk4\ui\Table();
$html = $table->render();
It works also without the App object, but then no CSS is applied.