.content{
	/*相对于谁？*/
	position: absolute;
	top:50%;
	left: 50%;
	width: 400px;
	height: 200px;
	margin-top: -100px;
	margin-left: -200px;
	background-color: #ccc;
}
.left_top{
	/*相对于谁*/
	position: absolute;
	top:0;
	left: 0;
	background-color: yellow;
	width: 50px;
	height: 50px;
	border-radius: 0 0 50px 0;
}
.right_bottom{
	position: absolute;
	bottom:0;
	right: 0;
	background-color: yellow;
	width: 50px;
	height: 50px;
	border-radius:50px 0 0 0;
}