<div class="background">
<div class="triangle"></div>
</div>
* {
margin: 0;
padding: 0;
outline: 0 none;
box-sizing: border-box;
}
.background {
width: 100vw;
min-height: 100vh;
display: block;
overflow-x: hidden;
}
.triangle {
background: #404040;
height: 100vh;
display: block;
position: relative;
}
.triangle:after {
content: '';
position: absolute;
bottom: -15rem;
width: 0;
height: 0;
border: 1rem solid transparent;
background: transparent;
border-bottom: 14rem solid #404040;
transform: rotateX(180deg) scaleX(42);
left: 48.8%;
}