<div class="corner"></div>
.corner{
width: 500px;
height: 150px;
background: #58a;
background: linear-gradient(-135deg,transparent 120px,pink 0);
}

image.png
<div class="corner"></div>
.corner{
width: 500px;
height: 150px;
background: #58a;
background: linear-gradient(-135deg,transparent 120px,pink 0);
}