JSBIN
HTML
<div class="container"><div class="triangle-up"><div></div></div></div>
CSS
.container{
width: 500px;
background-color: #000;
overflow: hidden;
}
.triangle-up {
height: 0;
padding-bottom: 24%;
}
.triangle-up div {
width: 0;
height: 0;
border-left: 900px solid #4679BD;
border-top: 60px solid transparent;
border-bottom: 60px solid transparent;
}