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.
itRenders('boolean prop with zero value', async render => {
        const e = await render(<div hidden={0} />);
        expect(e.getAttribute('hidden')).toBe(null);
      });
      itRenders('no boolean prop with null value', async render => {
        const e = await render(<div hidden={null} />);
        expect(e.hasAttribute('hidden')).toBe(false);
      });
    });
    describe('download property (combined boolean/string attribute)', function() {
      itRenders('download prop with true value', async render => {
        const e = await render(<a download={true} />);
        expect(e.getAttribute('download')).toBe('');
      });
      itRenders('download prop with false value', async render => {
        const e = await render(<a download={false} />);
        expect(e.getAttribute('download')).toBe(null);
      });
      itRenders('download prop with string value', async render => {
        const e = await render(<a download="myfile" />);
        expect(e.getAttribute('download')).toBe('myfile');
      });
      itRenders('download prop with string "false" value', async render => {
        const e = await render(<a download="false" />);
        expect(e.getAttribute('download')).toBe('false');
      });
      itRenders('download prop with string "true" value', async render => {
        const e = await render(<a download={'true'} />);
        expect(e.getAttribute('download')).toBe('true');
      });