How do I center div horizontally and vertically within a div using flexbox?

Question

How do I center (horizontally) and middle (vertically) a div within another div using flexbox?

Other Q/A exists, but are too specific

Answer 1

jsFiddle

In a flexbox, setting margin to auto will set the margin automatically for both the horizontal and vertical axis.

HTML

<div class="parent">
    <div class="child">
    </div>
</div>

CSS

html, body {width:100%;height:100%;}
.parent {
    display: flex;
    width: 100%;
    height: 100%;
    border: 5px solid red;
}

.child {
  width: 100px;
  height: 100px;
  margin: auto; /* Auto margin for both horizontal and vertical axis */
  border: 5px solid blue;
}