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.
var addendum2 = ReactDOMFeatureFlags.useFiber
        ? '\n    in tr (at **)' +
            '\n    in Row (at **)' +
            '\n    in table (at **)' +
            '\n    in Foo (at **)'
        : ' See Row > tr > #text.';
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(1)[0])).toBe(
        'Warning: validateDOMNesting(...): Text nodes cannot appear as a ' +
          'child of <tr>.' +
          addendum2,
      );
      var addendum3 = ReactDOMFeatureFlags.useFiber
        ? '\n    in table (at **)' + '\n    in Foo (at **)'
        : ' See Foo > table > #text.';
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(2)[0])).toBe(
        'Warning: validateDOMNesting(...): Whitespace text nodes cannot ' +
          "appear as a child of <table>. Make sure you don't have any extra " +
          'whitespace between tags on each line of your source code.' +
          addendum3,
      );
    });
    it('gives useful context in warnings', () => {
      spyOn(console, 'error');
      function Row() {
        return <tr />;
      }
      function FancyRow() {
        return <Row />;
      }
      class Table extends React.Component {