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.
render() {
          return <div ref={e => refCount++} />;
        }
      }
      await expectMarkupMatch(<RefsComponent />, <div />);
      expect(refCount).toBe(0);
    });
    it('should run ref code on client', async () => {
      let refCount = 0;
      class RefsComponent extends React.Component {
        render() {
          return <div ref={e => refCount++} />;
        }
      }
      await expectMarkupMatch(<div />, <RefsComponent />);
      expect(refCount).toBe(1);
    });
    it('should send the correct element to ref functions on client', async () => {
      let refElement = null;
      class RefsComponent extends React.Component {
        render() {
          return <div ref={e => (refElement = e)} />;
        }
      }
      const e = await clientRenderOnServerString(<RefsComponent />);
      expect(refElement).not.toBe(null);
      expect(refElement).toBe(e);
    });
    it('should have string refs on client when rendered over server markup', async () => {
      class RefsComponent extends React.Component {