<!DOCTYPE html>
<style>
.a { }
#f {
position: absolute;
left: 120px;
top: 120px;
width: 250px;
height: 250px;
border: 0;
outline: 100px solid #fc8;
}
</style>
<div class="a"></div>
<iframe id="f" srcdoc="
<style>
body { will-change: transform; }
#b { height: 200px; background: blue; opacity: 0.2; }
</style>
<div id=b></div>"></iframe>