html

<div id="text">Some text</div>
<pre id="log"></pre>

css

#text::after {
  content: "⭐";
  display: inline-block; /* Needed as the `transform` property does not apply to inline elements */
}
#text::before {
  content: "😊";
  display: inline-block;
}

js

const log = document.getElementById("log");
const text = document.getElementById("text");

// Create the keyframe and launch the animation
const animation = text.animate(
  [
    { transform: "rotate(0)" },
    { transform: "rotate(180deg)" },
    { transform: "rotate(360deg)" },
  ],
  { duration: 3000, iterations: Infinity, pseudoElement: "::after" },
);

// Get the value of KeyframeEffect.pseudoElement
function logPseudoElement() {
  const keyframeEffect = animation.effect;
  log.textContent = `Value of pseudoElement animated: ${keyframeEffect.pseudoElement}`;
  requestAnimationFrame(logPseudoElement);
}

// Every 6 seconds, switch the pseudo-element animated
function switchPseudoElement() {
  const keyframeEffect = animation.effect;
  keyframeEffect.pseudoElement =
    keyframeEffect.pseudoElement === "::after" ? "::before" : "::after";
  setTimeout(switchPseudoElement, 6000);
}

switchPseudoElement();
logPseudoElement();