in Anki Manual>> [Styling & HTML] >> Fading and Scrolling there is this code to fade in cards:
<script>qFade=100; if (typeof anki !== 'undefined') anki.qFade=qFade;</script>
it works perfectly fine with Version 2.1.44 on Intel Mac with 11.3.1 (20E241).
I tested every version since then plus the new beta2 2.1.50. but it does not work on those versions.