y set positive number, the actual value of test element size is same as the positive number">
<style>
  #container {
    background-color: red;
    display: flex;
    height: 100px;
    width: 100px;
  }
  #container div{
    background-color: green;
    height: 100px;
  }
  #test {
    flex-basis: 60px;
  }
  #ref {
    width: 40px;
  }
</style>
<body>
  <p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
  <div id="container">
    <div id="test"></div>
    <div id="ref"></div