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.
constructor(options) {
    super(options);
    this.buffer = '';
  }
  _write(chunk, encoding, cb) {
    this.buffer += chunk;
    cb();
  }
}
async function renderIntoStream(reactElement, errorCount = 0) {
  return await expectErrors(
    () =>
      new Promise(resolve => {
        let writable = new DrainWritable();
        ReactDOMServer.renderToNodeStream(reactElement).pipe(writable);
        writable.on('finish', () => resolve(writable.buffer));
      }),
    errorCount,
  );
}
async function streamRender(reactElement, errorCount = 0) {
  const markup = await renderIntoStream(reactElement, errorCount);
  var domElement = document.createElement('div');
  domElement.innerHTML = markup;
  return domElement.firstChild;
}
const clientCleanRender = (element, errorCount = 0) => {
  const div = document.createElement('div');