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.
expect(e.firstChild.tagName).toBe('BR');
    });
    if (ReactDOMFeatureFlags.useFiber) {
      itRenders('a string', async render => {
        let e = await render('Hello');
        expect(e.nodeType).toBe(3);
        expect(e.nodeValue).toMatch('Hello');
      });
      itRenders('a number', async render => {
        let e = await render(42);
        expect(e.nodeType).toBe(3);
        expect(e.nodeValue).toMatch('42');
      });
      itRenders('an array with one child', async render => {
        let e = await render([<div key={1}>text1</div>]);
        let parent = e.parentNode;
        expect(parent.childNodes[0].tagName).toBe('DIV');
      });
      itRenders('an array with several children', async render => {
        let Header = props => {
          return <p>header</p>;
        };
        let Footer = props => {
          return [<h2 key={1}>footer</h2>, <h3 key={2}>about</h3>];
        };
        let e = await render([
          <div key={1}>text1</div>,
          <span key={2}>text2</span>,
          <Header key={3} />,
          <Footer key={4} />,