<style>
div {
position: relative;
width: 100px;
height: 50px;
background-color: tomato;
}
div::after{
position: absolute;
right: 0px;
bottom: -10px;
content: "";
width: 0;
height: 0;
border-right: 10px solid transparent;
border-top: 10px solid tomato;
}
</style>
<div></div>