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.
warnedProperties[name] = true;
      return false;
    }
    return true;
  };
}
var warnUnknownProperties = function(type, props, debugID) {
  var unknownProps = [];
  for (var key in props) {
    var isValid = validateProperty(type, key, props[key], debugID);
    if (!isValid) {
      unknownProps.push(key);
    }
  }
  var unknownPropString = unknownProps.map(prop => '`' + prop + '`').join(', ');
  if (unknownProps.length === 1) {
    warning(
      false,
      'Invalid prop %s on <%s> tag. Either remove this prop from the element, ' +
        'or pass a string or number value to keep it in the DOM. ' +
        'For details, see https://fb.me/react-unknown-prop%s',
      unknownPropString,
      type,
      getStackAddendum(debugID),
    );
  } else if (unknownProps.length > 1) {
    warning(
      false,
      'Invalid props %s on <%s> tag. Either remove these props from the element, ' +
        'or pass a string or number value to keep them in the DOM. ' +