It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
var lowerCasedName = name.toLowerCase();
    var registrationName = EventPluginRegistry.possibleRegistrationNames.hasOwnProperty(
      lowerCasedName,
    )
      ? EventPluginRegistry.possibleRegistrationNames[lowerCasedName]
      : null;
    if (registrationName != null) {
      warning(
        false,
        'Unknown event handler property `%s`. Did you mean `%s`?%s',
        name,
        registrationName,
        getStackAddendum(debugID),
      );
      warnedProperties[name] = true;
      return true;
    }
    if (lowerCasedName.indexOf('on') === 0) {
      warning(
        false,
        'Unknown event handler property `%s`. It will be ignored.%s',
        name,
        getStackAddendum(debugID),
      );
      warnedProperties[name] = true;
      return true;
    }
    
    if (ARIA_NAME_REGEX.test(name)) {
      return true;