<!DOCTYPE html>
<link rel="help" href="https://drafts.fxtf.org/filter-effects-2/#BackdropFilterProperty">

<div class="red_square"></div>
<div class="filter"></div>

<style>
div {
  width: 100px;
  position: absolute;
}
body { overflow: hidden; margin: 0;}
.red_square {
  height: 100px;
  left:30px;
  top: 20px;
  background: red;
  transform: rotate(30deg);
}
.filter {
  height: 200px;
  background: transparent;
  backdrop-filter: blur(20px);
}
</style>