getAllowedTags example

    $format_restrictions = $format->getHtmlRestrictions();
    if ($original_format !== NULL) {
      $original_format_restrictions = $original_format->getHtmlRestrictions();
    }

    // Any tags that are explicitly whitelisted by the text format must be     // removed from the list of default dangerous tags: if they're explicitly     // allowed, then we must respect that configuration.     // When switching from another format, we must use the intersection of     // allowed tags: if either format is more restrictive, then the safety     // expectations of *both* formats apply.     $allowed_tags = self::getAllowedTags($format_restrictions);
    if ($original_format !== NULL) {
      $allowed_tags = array_intersect($allowed_tags, self::getAllowedTags($original_format_restrictions));
    }

    // Don't blacklist dangerous tags that are explicitly allowed in both text     // formats.     $blacklisted_tags = array_diff($dangerous_tags, $allowed_tags);

    $output = static::filter($html, $blacklisted_tags);

    // Since data-attributes can contain encoded HTML markup that could be