100% height with flex

HTML

<html>

  <body style="display: flex;">
    <div style="flex: 1; margin: 25px; background-color: black;"></div>
  </body>

</html>

CSS

html,
body {
  /* fix for IE11, not needed for chrome/ff */
  height: 100%;
  /* CSS-reset for chrome */
  margin: 0;
}