CSS3倒三角形

css：

.triangle-left {
    width:0;
    height:0;
    border-right:20px solid yellow;
    border-top:20px solid transparent;
    border-bottom:20px solid transparent;
}

.triangle-up {
    width:0;
    height:0;
    border-bottom:20px solid yellow;
    border-left:20px solid transparent;
    border-right:20px solid transparent;
}

.triangle-right {
    width:0;
    height:0;
    border-left:20px solid yellow;
    border-top:20px solid transparent;
    border-bottom:20px solid transparent;
}

.triangle-down {
    width:0;
    height:0;
    border-top:20px solid yellow;
    border-left:20px solid transparent;
    border-right:20px solid transparent;
}

效果预览，请运行下面代码

<!DOCTYPE html>
<html>
<head>
<meta charset="gb2312" />
<title>无标题文档</title>
<style>
.triangle-left {
width:0;
height:0;
border-right:20px solid yellow;
border-top:20px solid transparent;
border-bottom:20px solid transparent;
}

.triangle-up {
width:0;
height:0;
border-bottom:20px solid yellow;
border-left:20px solid transparent;
border-right:20px solid transparent;
}
.triangle-right {
width:0;
height:0;
border-left:20px solid yellow;
border-top:20px solid transparent;
border-bottom:20px solid transparent;
}
.triangle-down {
width:0;
height:0;
border-top:20px solid yellow;
border-left:20px solid transparent;
border-right:20px solid transparent;
}
</style>
</head>

<body>
<div class="triangle-left"></div>
<div class="triangle-up"></div>
<div class="triangle-right"></div>
<div class="triangle-down"></div>
</body>
</html>