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('a div with multiple children', async render => {
        const e = await render(
          <div id="parent"><div id="child1" /><div id="child2" /></div>,
        );
        expect(e.id).toBe('parent');
        expect(e.childNodes.length).toBe(2);
        expect(e.childNodes[0].id).toBe('child1');
        expect(e.childNodes[0].childNodes.length).toBe(0);
        expect(e.childNodes[1].id).toBe('child2');
        expect(e.childNodes[1].childNodes.length).toBe(0);
      });
      itRenders(
        'a div with multiple children separated by whitespace',
        async render => {
          const e = await render(
            <div id="parent"><div id="child1" /> <div id="child2" /></div>,
          );
          expect(e.id).toBe('parent');
          let child1, child2, textNode;
          if (ReactDOMFeatureFlags.useFiber) {
            
            
            expect(e.childNodes.length).toBe(3);
            child1 = e.childNodes[0];
            textNode = e.childNodes[1];
            child2 = e.childNodes[2];
          } else {