<!doctype HTML>

<style>
#container {
  contain: content;
  width: 150px;
  height: 150px;
  background: lightblue;
}
.child {
  width: 20px;
  height: 20px;
  background: lightgreen;
}
</style>

<div id="log">PASS</div>
<div id="container">
  <div class="child">1</div>
  <div class="child">2</div>
  <div class="child">3</div>
  <div class="child">4</div>
  <div class="child">5</div>
  <div class="child">6</div>
  <div class="child">7</div>
</div>