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.
},
            };
          },
        };
        let e = await render(threeDivIterable);
        let parent = e.parentNode;
        expect(parent.childNodes.length).toBe(3);
        expect(parent.childNodes[0].tagName).toBe('DIV');
        expect(parent.childNodes[1].tagName).toBe('DIV');
        expect(parent.childNodes[2].tagName).toBe('DIV');
      });
      itRenders('emptyish values', async render => {
        let e = await render(0);
        expect(e.nodeType).toBe(TEXT_NODE_TYPE);
        expect(e.nodeValue).toMatch('0');
        
        
        expect((await render(<div>{''}</div>)).textContent).toBe('');
        expect(await render([])).toBe(null);
        expect(await render(false)).toBe(null);
        expect(await render(true)).toBe(null);
        expect(await render(undefined)).toBe(null);
        expect(await render([[[false]], undefined])).toBe(null);
      });
    }
  });
  describe('property to attribute mapping', function() {
    describe('string properties', function() {
      itRenders('simple numbers', async render => {
        const e = await render(<div width={30} />);