<!DOCTYPE html>
<title>Reference</title>

<style>
  /* establish context */
  .container {
    clear: both;
    padding: 10px;
    width: 0;
  }

  span {
    display: inline-block;
    width: 20px;
    height: 20px;
    background-color: blue;
  }

  /* visualize size contribution */
  .container > div {
    float: left;
    border: solid orange 20px;
    border-style: none solid;
  }
  .container > div > div {
    border-right: solid 20px aqua;
    writing-mode: vertical-rl;
    width: 80px;
    height: 20px;
  }

  /* test width */
  /* content = 100% = 100px = 80px + border */
  /* choose sizes that are different than content to see if how they take effect */
  .container > div > .raw-percent {
    width: 50px;
  }
  .container > div > .calc-percent,
  .container > div > .no-percent {
    width: 40px;
  }
</style>

<!-- calculating min-content widths -->
<div class='container'>
  <div><div class="control"><span></span></div></div>
</div>
<div class='container'>
  <div style='width: 100px;'><div class="raw-percent"><span></span></div></div>
</div>
<div class='container'>
  <div style='width: 100px;'><div class="calc-percent"><span></span></div></div>
</div>
<div class='container'>
  <div><div class="no-percent"><span></span></div></div>
</div>