Why doesn't justify-content: center work in IE?

Question

I have this simple div with a button inside of it. justify-content: center; works fine using Firefox and Chrome, but does not work on IE 11:

#div {    height: 200px;    width: 50px;    border: 1px solid black;    display: flex;    flex: 0 0 auto;    align-items: center;    justify-content: center;  }  #button {    height: 50px;    width: 200px;    min-width: 200px;    border: 1px solid black;    background-color: red;  }

<div id="div">    <button id="button">HELLO</button>  </div>

My goal is that, when I use transform with rotate(90deg) or rotate(270deg), the button will fit into the div:

#div {    height: 200px;    width: 50px;    border: 1px solid black;    display: flex;    flex: 0 0 auto;    align-items: center;    justify-content: center;  }  #button {    height: 50px;    width: 200px;    min-width: 200px;    border: 1px solid black;    background-color: red;    transform: rotate(90deg);  }

<div id="div">    <button id="button">HELLO</button>  </div>

The height and width of the div and button are always the same, but are customizable.

As much as possible, I prefer not wrapping elements.

Answer 1

IE11 needs the parent to have flex-direction: column.

This example has your button rotated:

#div {    height: 200px;    width: 50px;    border: 1px solid black;    display: flex;    align-items: center;    justify-content: center;    flex-direction: column  }  #button {    height: 50px;    width: 200px;    min-width: 200px;    border: 1px solid black;    background-color: red;    transform: rotate(90deg);  }

<div id="div">    <button id="button">HELLO</button>  </div>

Answer 2

In my case I had to make the flex container's height 100%. justify-content worked without a problem after that.

I also had to make the (first level) children's max-width 100% to fix some content overflowing horizontally.