Just updated a local test site to 5.3.0-RC1 and noticed this JS error in front-end display:
Uncaught TypeError: Node.appendChild: Argument 1 is not an object.

Probably related to this line:
<script> (function(){ document.addEventListener('DOMContentLoaded',function(){ var ai = document.getElementsByClassName('article-index'); var target = document.getElementById('mod_article_index'); target.appendChild(ai[0]); }); })(); </script>

Info:
this test site has no fancy extension,
no change has been made to cassiopeia template nor to its index.php.
It's a kind of low-level benchmark for my tests.