firefox/testing/web-platform/tests/permissions-policy/resources/permissions-policy-idle-detection.html

<script>
'use strict';

new IdleDetector().start().then(() => {
  window.parent.postMessage({ type: 'availability-result', enabled: true }, '*');
}, error => {
  window.parent.postMessage({ type: 'availability-result', enabled: false }, '*');
});

</script>