css/css-anchor-position/anchor-center-visibility-change-ref.html

<!DOCTYPE html>
<style>
.container {
  width: 100px;
  height: 100px;
  border: solid 3px;
  position: relative;
  margin: 50px;
}

.anchor {
  position: relative;
  width: 50px;
  height: 50px;
  left: 40px;
  top: 5px;
  background: lime;
}

.target {
  position: fixed;
  background: cyan;
  left: 111px;
}
</style>

<div class="container">
  <div class="anchor"></div>
  <div class="target">
    <div style="width:30px;height:20px;"></div>
  </div>
</div>