css/css-shadow-parts/animation-part.html

<!doctype html>
<title>Animation name and ::part()</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="match" href="animation-part-ref.html">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1852834">
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/1995">
<wrapper-el></wrapper-el>
<script>
class WrapperEl extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: "open" });
    this.shadowRoot.innerHTML = `
      <style>
        @keyframes green {
          from { background: green; }
          to { background: green; }
        }

        display-el::part(icon) {
          animation: green 1s linear infinite;
        }
      </style>
      <display-el></display-el>
    `;
  }
}
customElements.define("wrapper-el", WrapperEl);

class DisplayEl extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: "open" });
    this.shadowRoot.innerHTML = `
      <style>
        div {
          width: 16px; height: 16px; background: red;
        }
      </style>
      <p><div part="icon"></div></p>
    `;
  }
}
customElements.define("display-el", DisplayEl);
</script>