final class FieldFilteredMarkup

final class FieldFilteredMarkup implements MarkupInterface, \Countable {
  use MarkupTrait;

  /**
   * Overrides \Drupal\Component\Render\MarkupTrait::create().
   *
   * @return string|\Drupal\Component\Render\MarkupInterface
   *   A safe string filtered with the allowed tag list and normalized.
   *
   * @see \Drupal\Core\Field\FieldFilteredMarkup::allowedTags()
   * @see \Drupal\Component\Utility\Xss::filter()
   * @see \Drupal\Component\Utility\Html::normalize()
   */
  public static function create($string) {
    $string = (string) $string;
    if ($string === '') {
      return '';
    }
    $safe_string = new static();

    // All known XSS vectors are filtered out by
    // \Drupal\Component\Utility\Xss::filter(), all tags in the markup are
    // allowed intentionally by the trait, and no danger is added in by
    // \Drupal\Component\Utility\HTML::normalize(). Since the normalized value
    // is essentially the same markup, designate this string as safe as well.
    // This method is an internal part of field sanitization, so the resultant,
    // sanitized string should be printable as is.
    $safe_string->string = Html::normalize(Xss::filter($string, static::allowedTags()));
    return $safe_string;
  }

  /**
   * Returns the allowed tag list.
   *
   * @return string[]
   *   A list of allowed tags.
   */
  public static function allowedTags() {
    return [
      'a',
      'b',
      'big',
      'code',
      'del',
      'em',
      'i',
      'ins',
      'pre',
      'q',
      'small',
      'span',
      'strong',
      'sub',
      'sup',
      'tt',
      'ol',
      'ul',
      'li',
      'p',
      'br',
      'img',
    ];
  }

  /**
   * Returns a human-readable list of allowed tags for display in help texts.
   *
   * @return string
   *   A human-readable list of allowed tags for display in help texts.
   */
  public static function displayAllowedTags() {
    return '<' . implode('> <', static::allowedTags()) . '>';
  }

}