 of testCodePoints) {
    for (const separatorMode of [0, 1, 2]) {
      const str = createStringWithSeparatorMode(
        codePoint, prefix, separatorMode);
      if (isValidCustomElementName(str)) {
        validCustomElementNames.push(str);
      } else {
        invalidCustomElementNames.push(str);
      }
    }
  }
}

let nextClassNumber = 1;
function createElementClass() {
  const name = `CustomElement${nextClassNumber++}`;
  const newClass = function() {};
  newClass.prototype = HTMLElement;
  retur