You create a custom controller and want to show a table that resembles the core back-end tables. This snippet shows you how to do it.
use Drupal\Core\Render\Markup;
$rows = [
[Markup::create('<strong>test 1</strong>'),'test'],
[Markup::create('<s>test 2</s>'), 'test'],
[Markup::create('<div>test 3</div>'), 'test'],
];
$header = [
'title' => t('Title'),
'content' => t('Content'),
];
$build['table'] = [
'#type' => 'table',
'#header' => $header,
'#rows' => $rows,
'#empty' => t('No content has been found.'),
];
return [
'#type' => '#markup',
'#markup' => render($build)
];
The result looks like this: