<!DOCTYPE html>
<html>
<head>
<style>
  .outer {
    overflow:hidden;
    border-radius: 1px;
    display:flex;
    backface-visibility: hidden;
  }
  #child-overflow { 
    border-radius: 5px;
    overflow:hidden;
    background-color: lightblue;
    width: 100px;
    height: 40px;
    z-index:1
  }
  #sibling {
    background-color: green;
    width: 100px;
    height: 40px;
    z-index:1
  }
</style>
</head>
<div class="outer">
    <div id="child-overflow"></div>
    <div id="sibling"></div>
</div>