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.
/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @emails react-core
 */
'use strict';
describe('ReactDOM unknown attribute', () => {
  var React;
  var ReactDOM;
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
  });
  function normalizeCodeLocInfo(str) {
    return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
  }
  function testUnknownAttributeRemoval(givenValue) {
    var el = document.createElement('div');
    ReactDOM.render(<div unknown="something" />, el);
    expect(el.firstChild.getAttribute('unknown')).toBe('something');
    ReactDOM.render(<div unknown={givenValue} />, el);
    expect(el.firstChild.hasAttribute('unknown')).toBe(false);
  }
  function testUnknownAttributeAssignment(givenValue, expectedDOMValue) {