如何在运行时更改元素样式？

element.style.property = value;

element.style.backgroundColor = 'red';

var style = document.createElement('style');
style.innerHTML = 'selector { property: value; }';
document.head.appendChild(style);

var style = document.createElement('style');
style.innerHTML = '.my-class { background-color: red; }';
document.head.appendChild(style);

element.classList.add('class-name');

element.classList.add('my-class');

@keyframes my-animation {
  0% { property: value1; }
  100% { property: value2; }
}

element {
  animation: my-animation 1s;
}

@keyframes fade-in {
  0% { opacity: 0; }
  100% { opacity: 1; }
}

element {
  animation: fade-in 1s;
}