SOURCE

<div class="phone-app">
	<p>手机APP, 扫描二维码</p>
</div>

/*利用伪类:after 实现三角形*/
.phone-app {
	position: relative;
	width: 200px;
	height: 45px;
	line-height: 45px;
	background-color: #c91717;
	font-weight: bold;
	color: #FFF;
	text-align: center;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	border-radius: 5px;
}
/*优点 无多余标签、代码简洁*/
.phone-app:after {
	content: "";
	width: 0;
	height: 0;
	border: 17px solid transparent;
	border-top-color:#c91717;
	position: absolute; /*定位*/
	top: 45px;
	left: 83px;
}