<ul class="flex-container">
<li class="flex-item">1</li>
<li class="flex-item">2</li>
<li class="flex-item">3</li>
<li class="flex-item">4</li>
</ul>
.flex-container {
padding: 0;
margin: 0;
list-style: none;
-ms-box-orient: horizontal;
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -moz-flex;
display: -webkit-flex;
display: flex;
-webkit-justify-content: space-around;
justify-content: space-around;
-webkit-flex-flow: row wrap;
flex-flow: row wrap;
-webkit-align-items: stretch;
align-items: stretch;
}
.flex-item:nth-of-type(1) { flex-grow: 1; }
.flex-item:nth-of-type(2) { flex-grow: 1; }
.flex-item:nth-of-type(3) { flex-grow: 2; }
.flex-item:nth-of-type(4) { flex-grow: 1; }
.flex-item:nth-of-type(5) { flex-grow: 1; }
.flex-item {
background: tomato;
border: 3px solid rgba(0,0,0,.2);
line-height: 100px;
color: white;
font-weight: bold;
font-size: 2em;
text-align: center;
}
This Pen doesn't use any external CSS resources.