Import fiddle as a JS Module - chain fiddles together

HTML

<button popovertarget="popover">Open Popover</button>
<div popover id="popover">Greetings, one and all!</div>

CSS

#popover::backdrop {
  background: black;
  opacity: .5;
}

#popover {
  transition: background 3s;
  background: red;
}

#popover:popover-open {
  @starting-style {
    background: blue;  
  }
}