SOURCE

<div class="icon-box">
	<i class="icon-triangle"></i>
</div>

.icon-box {
    border: 3px solid green;
    border-radius: 3px;
}
.icon-triangle {
    display: block;
    position: relative;
    width: 0;
    height: 0;
    border-width: 0 50px 50px;
    border-color: transparent transparent black;
    border-style: solid;
}

.icon-triangle:after {
    content: '';
    background-color: yellow;
    position: absolute;
    border-width: 0 40px 40px;
    border-color: transparent transparent red;
    border-style: solid;
    top: 50%;
    left: 50%;
    transform: translateX(-50%) translateY(-50%);
}