code: public static SanitizedContent cleanHtml(
        SoyValue value, Collection<? extends OptionalSafeTag> optionalSafeTags) {
      value = normalizeNull(value);
      Dir valueDir = null;
      if (value instanceof SanitizedContent) {
        SanitizedContent sanitizedContent = (SanitizedContent) value;
        if (sanitizedContent.getContentKind() == SanitizedContent.ContentKind.HTML) {
          return (SanitizedContent) value;
        }
        valueDir = sanitizedContent.getContentDirection();
      }
      return cleanHtml(value.coerceToString(), valueDir, optionalSafeTags);
    }
docs: Normalizes the input HTML while preserving "safe" tags and the known directionality.
  
  @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}