code: public static SanitizedContent cleanHtml(
        String value, Collection<? extends OptionalSafeTag> optionalSafeTags) {
      return cleanHtml(value, null, optionalSafeTags);
    }
docs: Normalizes the input HTML while preserving "safe" tags. The content directionality is unknown.
  
  @param optionalSafeTags to add to the basic whitelist of formatting safe tags
  @return the normalized input, in the form of {@link SanitizedContent} of {@link
  ContentKind#HTML}