[Links] #d8

1.md

From How to add classes to links in Drupal 8

$url = Url::fromUri('http://drupal.org');
$link_options = array(
  'attributes' => array(
    'class' => array(
      'my-first-class',
      'my-second-class',
    ),
  ),
);
$url->setOptions($link_options);
$link = \Drupal::l(t('Link title'), $url);

note:

l() is deprecated, so use Link::fromTextAndUrl(); instead