HTML in menu items D8

.theme.php

<?php

/**
 * Implements hook_preprocess_menu().
 */
function THEME_preprocess_menu(&$vars, $hook) {
  if ($hook == 'menu__main') {
    $items = $vars['items'];
    foreach ($items as $key => $item) {
      $original_title = $vars['items'][$key]['title'];
      $vars['items'][$key]['title'] = array(
        '#markup' => $original_title,
        '#allowed_tags' => array('span', 'b', 'i'),
      );
    }
  }
}