三角区域

#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>