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.
return renderIntoDom(element, div, false, errorCount);
};
const clientRenderOnServerString = async (element, errorCount = 0) => {
  const markup = await renderIntoString(element, errorCount);
  resetModules();
  var domElement = document.createElement('div');
  domElement.innerHTML = markup;
  let serverNode = domElement.firstChild;
  const firstClientNode = await renderIntoDom(
    element,
    domElement,
    true,
    errorCount,
  );
  let clientNode = firstClientNode;
  
  while (serverNode || clientNode) {
    expect(serverNode != null).toBe(true);
    expect(clientNode != null).toBe(true);
    expect(clientNode.nodeType).toBe(serverNode.nodeType);
    
    
    
    expect(serverNode === clientNode).toBe(true);
    serverNode = serverNode.nextSibling;
    clientNode = clientNode.nextSibling;
  }
  return firstClientNode;
};
function BadMarkupExpected() {}