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.
itThrows(
    `when rendering ${desc} with server string render`,
    () => testFn(serverRender),
    partialMessage,
  );
  itThrows(
    `when rendering ${desc} with clean client render`,
    () => testFn(clientCleanRender),
    partialMessage,
  );
  
  
  itThrows(
    `when rendering ${desc} with client render on top of bad server markup`,
    () =>
      testFn((element, warningCount = 0) =>
        clientRenderOnBadMarkup(element, warningCount - 1),
      ),
    partialMessage,
  );
}
async function testMarkupMatch(serverElement, clientElement, shouldMatch) {