<main>
  <div>
     I'm a block-level element with an unknown height, centered vertically within my parent.
  </div>
</main>

body {
  background: #f06d06;
  font-size: 80%;
}

main {
  background: white;
  height: 300px;
  margin: 20px;
  width: 300px;
  position: relative;
  padding: 20px;
  display: table;
}
main div {
  background: black;
  color: white;
  padding: 20px;
  display: table-cell;
  vertical-align: middle;
}