<button popovertarget="mypopover">Toggle the popover</button>
<div id="mypopover" popover>Popover content</div>

[popover] {
  background: lightblue;
  padding: 20px;
}

[popover]:-internal-popover-in-top-layer::backdrop {
  background: rgba(0, 0, 0, .5);  
}