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.
expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('CUSTOM-TAG'));
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('IMG'));
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<IMG /> is using uppercase HTML.',
      );
    });
    it('should warn on props reserved for future use', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div aria="hello" />);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'The `aria` attribute is reserved for future use in React. ' +
          'Pass individual `aria-` attributes instead.',
      );
    });
    it('should warn if the tag is unrecognized', () => {
      spyOn(console, 'error');
      let realToString;
      try {
        realToString = Object.prototype.toString;
        let wrappedToString = function() {
          
          if (this instanceof window.HTMLUnknownElement) {
            return '[object HTMLUnknownElement]';
          }
          
          if (this instanceof window.HTMLTimeElement) {