方案一
.x {
border: solid 1px red;
width: 100px;
height: 100px;
position: relative;
background-color: transparent;
}
.x:before {
position: absolute;
top: 0px;
right:;
left:;
bottom:;
border-bottom: 100px solid red;
border-left: 100px solid transparent;
content: "";
}
.x:after {
position: absolute;
left: 1px;
right:;
top: 1px;
bottom:;
border-bottom: 99px solid white;
border-left: 99px solid transparent;
content: "";
}
.f {
border: solid 1px red;
width: 100px;
height: 100px;
position: relative;
background-color: transparent;
}
.f:before {
position: absolute;
top: 0px;
right:;
left:;
bottom:;
border-bottom: 100px solid red;
border-right: 100px solid transparent;
content: "";
}
.f:after {
position: absolute;
left:;
right: 1px;
top: 1px;
bottom:;
border-bottom: 99px solid white;
border-right: 99px solid transparent;
content: "";
}
x 是 斜线， f 是反斜线。
图：
附：CSS 图形：
css3激发想象/css3各种图形 http://www.cnblogs.com/wwcherish/archive/2012/09/18/2690336.html
CSS3机器猫： http://keleyi.com/keleyi/phtml/html5/3.htm
方案二
background-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="100%" height="100%"><line x1="0" y1="0" x2="100%" y2="100%" stroke="black" stroke-width="1"/></svg>');