<!DOCTYPE html>
<title>Reference: Select element in flexbox with zero height</title>
<link rel="author" title="Mozilla" href="https://mozilla.org">
<style>
  .container {
    height: 0px;
    border: 1px dotted black;
  }
  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 style="width:100%"><option>Hi</option></select>
  </div>
</body>