How to target Elements in a HTML Widget

It looks like you’re trying to play a video in its own <video> element:

<video id="myVideo" poster="${resourcesFolderName}/weltall_preview.jpg" playsinline="" width="100%" height="100%" preload="auto">
      <source src="${resourcesFolderName}/Weltall_2.mp4" type="video/mp4">
</video>

Instead of placing this in a HTML widget, you can place this in a regular rectangle and avoid all the complexity here. This will allow you to just use this code to play it:

var theVideo = document.getElementById("myVideo");
theVideo.play();

A bit more thorough information on this here: Creating Play & Pause Buttons for Video