Demo下载 效果如图：
css：
#box {
width: 280px;
height: 150px;
background: #ff6565;
padding: 20px;
position: relative;
overflow: hidden;
}
#content {
width: 280px;
height: 150px;
background: #ffe765;
}
#shadow {
width: 63px;
height: 63px;
position: absolute;
transform: rotate(-45deg);
top: -20px;
left: -20px;
background: url(shadow.png) no-repeat center bottom;
}
#shadow div {
width: 63px;
height: 60px;
background: #ff6565;
}
html：
<div id="box">
<div id="shadow"><div></div></div>
<div id="content"></div>
</div>
