LayoutTests/fast/dynamic/block-inside-span-gets-removed.html

<style>
div {
  font-family: Ahem;
  font-size: 20px;
}
</style>
<div>
  X<span>
   <div class=replace_this>Y</div>
 </span>
 <div></div>
</div>

<div>
 <div></div>
  X<span>
   <div class=replace_this>Y</div>
 </span>
</div>

<div>
  X<span>
   <div class=replace_this>Y</div>
 </span>
 <div></div>
 <div></div>
</div>

<div>
  X<span>
    <span>
      <div class=replace_this>Y</div>
    </span>
 </span>
</div>

<div>
  <span>
   <div class=replace_this>Y</div>
 </span>X
</div>
<script>
document.body.offsetHeight;
[...document.getElementsByClassName("replace_this")].forEach(replaceThis => {
  replaceThis.replaceWith(document.createElement("span").textContent = "X");
});
</script>