How can I vertically center rotated text using Flexbox layout?

Question

How can I vertically center rotated text using flexbox layout? I want something that looks like this:

Here's what I have so far:

html, body { height: 100%; }
body { background-color: #efefef; }

body > div {
  align-content: center;
  background-color: white;
  border: 1px solid black;
  display: flex;
  height: 100%;
  width: 25px;
}

body > div > div {
  flex: 1;
  transform: rotate(-90deg);
}

<div>
  <div>
    Where did I go?
  </div>
</div>

Answer 1

Add white-space: nowrap and center horizontally and vertically using:

align-items: center;
justify-content: center;

(and you don't need the flex: 1!)

Also removed the browser margin and added in box-sizing: border-box to add the finishing touches.

See demo below:

* {
  box-sizing: border-box;
}
html,
body {
  height: 100%;
  margin: 0;
}
body {
  background-color: #efefef;
}
body > div {
  background-color: white;
  border: 1px solid black;
  display: flex;
  height: 100%;
  width: 25px;
  align-items: center;
  white-space: nowrap;
  justify-content: center;
}
body > div > div {
  transform: rotate(-90deg);
}

<div>
  <div>
    Where did I go?
  </div>
</div>