在JavaScript中分配样式表会创建好的html,但不会显示样式

document.getElementById("myElement").style.color = "red";
document.getElementById("myElement").style.fontSize = "16px";

document.getElementById("myElement").classList.add("myClass");

.myClass {
  color: red;
  font-size: 16px;
}

document.getElementById("myElement").setAttribute("style", "color: red; font-size: 16px;");