非纯色背景下切角矩形的效果图如下：
HTML：
CSS：
.top-box {
position: relative;
display: block;
width: 90%;
min-height: 230px;
margin: 0 auto 40px;
padding: 0 10px 80px;
background: #fff;
background: linear-gradient(-135deg, transparent 20px, #FFF 0); //切角效果
box-sizing: border-box;
}
/* 左下角小三角 */
.top-box:before, .top-box:after {
position: absolute;
left: -2px;
bottom: -30px;
display: inline-block;
content: '';
height: 0;
border-left: 15px solid #0B113C;
border-top: 15px solid #0B113C;
border-right: 15px solid transparent;
border-bottom: 15px solid transparent;
}
.top-box:after {
left: 0;
bottom: -26px;
border-left: 15px solid #fff;
border-top: 15px solid #fff;
}
/* 切角矩形描边 */
.top-box-border {
position: absolute;
top: -2px;
bottom: -2px;
left: -2px;
right: -2px;
content: '';
background: #000;
background: linear-gradient(-135deg, transparent 20px, #0B113C 0);
z-index: -1;
}
