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>