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('an input value overriding defaultValue', async render => {
        const e = await render(
          <input value="foo" defaultValue="bar" readOnly={true} />,
          1,
        );
        expect(e.value).toBe('foo');
        expect(e.getAttribute('value')).toBe('foo');
        expect(e.getAttribute('defaultValue')).toBe(null);
      });
      itRenders(
        'an input value overriding defaultValue no matter the prop order',
        async render => {
          const e = await render(
            <input defaultValue="bar" value="foo" readOnly={true} />,
            1,
          );
          expect(e.value).toBe('foo');
          expect(e.getAttribute('value')).toBe('foo');
          expect(e.getAttribute('defaultValue')).toBe(null);
        },
      );
    });
    describe('checkboxes', function() {
      itRenders('a checkbox that is checked with an onChange', async render => {
        const e = await render(
          <input type="checkbox" checked={true} onChange={() => {}} />,
        );
        expect(e.checked).toBe(true);
      });
      itRenders('a checkbox that is checked with readOnly', async render => {