<!DOCTYPE html>
<title>Reference: Select element with background color in flexbox with zero height</title>
<link rel="author" title="Mozilla" href="https://mozilla.org">
<style>
  .container {
    height: 0px;
    border: 1px dotted black;
  }
  .with-background {
    width: 100%;
    background: lime;
  }
  select {
    /*
      WebKit applies intrinsic (default) margins to control elements in some circumstances,
      let's disable them to avoid mismatch errors caused by those margins.
    */
    margin: 0;
  }
</style>
<body>
  <div class="container">
    <select class="with-background"><option>Hi</option></select>
  </div>
</body>