SOURCE

<div class="container">
    <div class="box active">
        this is box one
    </div>
    <div class="box">
        this is box two
    </div>
</div>

<div class="angle"></div>

.container{
    display: flex;
    justify-content: space-around;
}
.box{
    border: 1px solid #ccc;
    width: 200px;
    height: 200px;
    line-height: 200px;
    text-align: center;
    border-radius: 4px;
}

.active{
    border-color: red;
    position: relative
}

.active::before{
    content: '';
    position: absolute;
    right: 0;
    bottom: 0;
    width: 0px;
    height: 0px;
    border: 20px solid;
    border-color:  transparent red red transparent;
}