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.
});
    });
    describe('null, false, and undefined children', function() {
      itRenders('null single child as blank', async render => {
        const e = await render(<div>{null}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('false single child as blank', async render => {
        const e = await render(<div>{false}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('undefined single child as blank', async render => {
        const e = await render(<div>{undefined}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('a null component children as empty', async render => {
        const NullComponent = () => null;
        const e = await render(<div><NullComponent /></div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          expect(e.childNodes.length).toBe(0);
        } else {
          
          
          expect(e.childNodes.length).toBe(1);
          expectEmptyNode(e.firstChild);
        }
      });
      itRenders('null children as blank', async render => {
        const e = await render(<div>{null}foo</div>);