<!doctype HTML>

<style>
#container {
  contain: content;
  width: 150px;
  height: 150px;
  background: lightblue;
}
#child1 {
  width: 50px;
  height: 50px;
  background: lightgreen;
}
#child2 {
  width: 50px;
  height: 50px;
  background: lightyellow;
}
</style>

<div id="log">PASS</div>
<div id="container">
  <div id="child1"></div>
  <div id="child2"></div>
</div>