<main>
  
  <div>
     I'm a block-level-ish element of an unknown width and height, centered vertically within my parent via Flexbox properties.
  </div>
  
</main>

body {
  background: #40BDB7;
  font-size: 80%;
  padding: 20px;
}

main {
  background: white;
  height: 200px;
  width: 60%;
  margin: 0 auto;
  padding: 20px;
  display: flex;
  justify-content: center;
  align-items: center;
  resize: both;
  overflow: auto;
  border-radius: 5px;
}

main div {
  background: black;
  color: white;
  width: 50%;
  padding: 20px;
  resize: both;
  overflow: auto;
  border-radius: 5px;
}