css/css-display/display-contents-dynamic-pseudo-insertion-001.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Test: Dynamic insertion on empty display: contents element with pseudo-elements</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="help" href="https://drafts.csswg.org/css-display-3/#valdef-display-contents">
<link rel=match href="display-contents-dynamic-pseudo-insertion-001-ref.html">
<style>
.contents {
  display: contents;
  border: 10px solid red;
}
.contents::before {
  content: "A";
}
.contents::after {
  content: "SS";
}
</style>
<div class="contents"></div>
<script>
document.body.offsetTop;
let span = document.createElement('span');
span.innerHTML = "P";
let contents = document.querySelector('.contents');
contents.parentNode.insertBefore(span, contents);
</script>