code: private static void checkLinkDependentAttributes(
        @Nullable String relValue, AttributeContract hrefValueContract) {
  
      if (hrefValueContract.equals(AttributeContract.SAFE_URL)
          && relValue != null
          && !LINK_HREF_SAFE_URL_REL_WHITELIST.contains(relValue.toLowerCase(Locale.ENGLISH))) {
        throw new IllegalArgumentException(
            "SafeUrl values for the href attribute are not allowed on <link rel="
                + relValue
                + ">. Did you intend to use a TrustedResourceUrl?");
      }
    }
docs: Checks that this combination of rel value and href contract is safe.
  
  @param relValue is the value of rel or null if rel isn't present.
  @throws IllegalArgumentException if this value and contract combination is not allowed.