Add this style:
.inner {
flex-direction: column;
}
That tells the flexbox to display its children in rows instead of columns. (I know, weird, right?)
Updated Snippet:
.container {
height: 200px;
width: 200px;
background: cornflowerblue;
position: relative;
}
.inner {
height: 100%;
position: absolute;
left: 0;
width: 100%;
top: 0;
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
align-items: center;
justify-content: center;
flex-direction: column;
}
.square {
width: 30px;
height: 30px;
background: tomato;
display: block;
}
<div class="container">
<div class="inner">
<div class="square"></div>
<p>some text</p>
</div>
</div>