chromium/third_party/blink/web_tests/external/wpt/css/printing/animations-shadow-print.html

<!DOCTYPE HTML>
<html>
<link rel="help" href="https://drafts.csswg.org/css-animations/">
<link rel="match" href="animations-shadow-print-ref.html">
<title>Static CSS animation in Shadow DOM</title>
<style>

@keyframes a {
  from, to { color: blue }
}

.anim {
  color: olive;
  animation: a 1s infinite;
}

</style>
<p class="anim">blue with animation support; olive without</p>
<div id="holder"></div>
<script>
let shadow_style = style = document.createElement("style");
shadow_style.textContent = "@keyframes shadow_a { from, to { color: orange } }";

let shadow = document.getElementById("holder").attachShadow({mode:"open"});
let shadow_p = document.createElement("p");

shadow_p.style.color = "olive";
shadow_p.style.animation = "shadow_a 1s infinite";
shadow_p.innerHTML = "orange with animation support; olive without";
shadow.appendChild(shadow_style);
shadow.appendChild(shadow_p);
</script>