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