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.
expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
            expectTextNode(e.childNodes[1], '<span>Text2&quot;</span>');
          }
        } else {
          
          
          expect(e.childNodes.length).toBe(6);
          expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
          expectTextNode(e.childNodes[3], '<span>Text2&quot;</span>');
        }
      });
    });
    describe('components that throw errors', function() {
      itThrowsWhenRendering(
        'a function returning undefined',
        async render => {
          const UndefinedComponent = () => undefined;
          await render(<UndefinedComponent />, 1);
        },
        ReactDOMFeatureFlags.useFiber
          ? 'UndefinedComponent(...): Nothing was returned from render. ' +
              'This usually means a return statement is missing. Or, to ' +
              'render nothing, return null.'
          : 'A valid React element (or null) must be returned. ' +
              'You may have returned undefined, an array or some other invalid object.',
      );
      itThrowsWhenRendering(
        'a class returning undefined',
        async render => {
          class UndefinedComponent extends React.Component {