Add custom classes to node links
In your theme open file THEMENAME.theme, example in my case THEMENAME is bartik and file is bartik.theme add code below:

/**
 * Implements hook_preprocess_links().
 */
function bartik_preprocess_links(&amp;$variables) {
  if (isset($variables['links']['node-readmore'])) {
    $variables['links']['node-readmore']['link']['#options']['attributes']['class'][] = 'css-readmore';
  }

  if (isset($variables['links']['comment-add'])) {
    $variables['links']['comment-add']['link']['#options']['attributes']['class'][] = 'css-comment';
  }

}


Update:

Code above will add css class to any readmore link, to add for article only you can add preprocess and check type is article.

/**
 * Implements hook_preprocess_HOOK() for node.html.twig.
 */
function bartik_preprocess_node(&amp;$variables) {
  ...

  if ($variables['teaser'] &amp;&amp; $variables['node']-&gt;getType() == 'article') {
    $node_type_is_article = &amp;drupal_static('node_type_is_article');
    $node_type_is_article = TRUE;
  }
}

/**
 * Implements hook_preprocess_links().
 */
function bartik_preprocess_links(&amp;$variables) {
  if ($node_type_is_article = &amp;drupal_static('node_type_is_article')) {
    if (isset($variables['links']['node-readmore'])) {
      $variables['links']['node-readmore']['link']['#options']['attributes']['class'][] = 'css-readmore';
    }

    if (isset($variables['links']['comment-add'])) {
      $variables['links']['comment-add']['link']['#options']['attributes']['class'][] = 'css-comment';
    }
  }
}


Clear all cache and see result. Here is my result: