code: public static String escapeHtmlAttribute(SoyValue value) {
      value = normalizeNull(value);
      if (isSanitizedContentOfKind(value, SanitizedContent.ContentKind.HTML)) {
        // |escapeHtmlAttribute should only be used on attribute values that cannot have tags.
        return stripHtmlTags(value.coerceToString(), null, true);
      }
      return escapeHtmlAttribute(value.coerceToString());
    }
docs: Converts the input to HTML by entity escaping, stripping tags in sanitized content so the
  result can safely be embedded in an HTML attribute value.