#triangle
{
border: 2px solid #999;
position: relative;
width: 50px;
height: 25px;
}
#triangle > div
{
border-style: solid;
border-color: transparent #900;
border-width: 25px 50px;
position: absolute;
left: 0;
top: 0;
clip: rect(0, 50px, 25px 0);
}
<div id="triangle">
<div></div>
</div>