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.
lowerCasedName === 'is' &&
      value !== null &&
      value !== undefined &&
      typeof value !== 'string'
    ) {
      warning(
        false,
        'Received a `%s` for string attribute `is`. If this is expected, cast ' +
          'the value to a string.%s',
        typeof value,
        getStackAddendum(debugID),
      );
      warnedProperties[name] = true;
      return true;
    }
    if (typeof value === 'number' && isNaN(value)) {
      warning(
        false,
        'Received NaN for numeric attribute `%s`. If this is expected, cast ' +
          'the value to a string.%s',
        name,
        getStackAddendum(debugID),
      );
      warnedProperties[name] = true;
      return true;
    }
    
    if (possibleStandardNames.hasOwnProperty(lowerCasedName)) {
      var standardName = possibleStandardNames[lowerCasedName];
      if (standardName !== name) {