css - box-sizing: border-box with no declared height/width

Question

I'm trying to understand how box-sizing: border-box work in the code below. When the height or width is set (no padding), it works as intended (border appears inside the div). But if you only use the padding to create the dimension of the div, it does not work. Can someone explain why? Here's the demo:

div.test {
  background-color: red;
  box-sizing: border-box;
  display: inline-block;
  border: 5px solid;
  text-align: center;
  padding: 50px;
  vertical-align: middle;
  // height: 100px;
  // width: 100px;
}

div.test:first-of-type {
  border: none;
}

<div class="test">aa</div>
<div class="test">aa</div>

Answer 1

TL;DR

An idea is to keep border for both. Instead of none simply make the color transparent so that the size (including border + padding) will always be the same for both.

div.test {
  background-color: red;
  box-sizing: border-box;
  display: inline-block;
  border: 5px solid;
  text-align: center;
  padding: 50px;
}

div.test:first-of-type {
  border-color: transparent;
}

<div class="test">aa</div>
<div class="test">aa</div>