css/css-anchor-position/anchor-position-top-layer-005.html

<!DOCTYPE html>
<title>Non-top-layer element cannot anchor to top-layer element</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-1/#target-anchor-element">
<link rel="author" href="mailto:xiaochengh@chromium.org">
<link rel="match" href="anchor-position-top-layer-ref.html">

<style>
#anchor {
  position: absolute;
  top: 300px;
  left: 200px;
  width: 100px;
  height: 100px;
  background: orange;
  anchor-name: --a;
}

#target {
  position: fixed;
  top: anchor(bottom, 200px);
  left: anchor(left, 300px);
  width: 100px;
  height: 100px;
  background: lime;
  position-anchor: --a;
}

body {
  margin: 0;
  height: 300vh;
}

dialog {
  margin: 0;
  border: 0;
  padding: 0;
  inset: auto;
  outline: none;
}

dialog::backdrop {
  background: transparent;
}
</style>

<dialog id="anchor"></dialog>
<div id="target"></div>

<script>
anchor.showModal();
document.scrollingElement.scrollTop = 100;
</script>