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.
<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];
      
      
      expectDev(parseInt(previousLine, 10) + 2).toBe(parseInt(currentLine, 10));
    });
    it('gives source code refs for unknown prop warning for exact elements in composition', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      class Parent extends React.Component {
        render() {
          return <div><Child1 /><Child2 /><Child3 /><Child4 /></div>;
        }
      }
      class Child1 extends React.Component {
        render() {
          return <div class="paladin">Child1</div>;
        }
      }
      class Child2 extends React.Component {