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.
});
    it('gives source code refs for unknown prop warning for exact elements', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        <div className="foo1">
          <div class="foo2" />
          <div onClick={() => {}} />
          <div onclick={() => {}} />
          <div className="foo5" />
          <div className="foo6" />
        </div>,
      );
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('className');
      var matches = console.error.calls.argsFor(0)[0].match(/.*\(.*:(\d+)\).*/);
      var previousLine = matches[1];
      expectDev(console.error.calls.argsFor(1)[0]).toContain('onClick');
      matches = console.error.calls.argsFor(1)[0].match(/.*\(.*:(\d+)\).*/);
      var currentLine = matches[1];
      
      
      expect(parseInt(previousLine, 10) + 2).toBe(parseInt(currentLine, 10));
    });
    it('gives source code refs for unknown prop warning for exact elements (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(
        <div className="foo1">
          <div class="foo2" />
          <div onClick="foo3" />
          <div onclick="foo4" />