.triangle-up {
    width: 0;
    height: 0;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid red;
}
.triangle-down {
    width: 0;
    height: 0;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-top: 100px solid red;
}

.triangle-left {
    width: 0;
    height: 0;
    border-right: 100px solid red;
    border-top: 50px solid transparent;
    border-bottom: 50px solid transparent;
}

.triangle-right {
    width: 0;
    height: 0;
    border-left: 100px solid red;
    border-top: 50px solid transparent;
    border-bottom: 50px solid transparent;
}