CSS实现DIV三角形

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#triangle-up {
    width: 0;
    height: 0;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid red;
}

#triangle-down {
    width: 0;
    height: 0;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-top: 100px solid red;
}

#triangle-left {
    width: 0;
    height: 0;
    border-top: 50px solid transparent;
    border-right: 100px solid red;
    border-bottom: 50px solid transparent;
}

#triangle-right {
    width: 0;
    height: 0;
    border-top: 50px solid transparent;
    border-left: 100px solid red;
    border-bottom: 50px solid transparent;
}

#triangle-topleft {
    width: 0;
    height: 0;
    border-top: 100px solid red;
    border-right: 100px solid transparent;
}

#triangle-topright {
    width: 0;
    height: 0;
    border-top: 100px solid red;
    border-left: 100px solid transparent;
}

#triangle-bottomleft {
    width: 0;
    height: 0;
    border-bottom: 100px solid red;
    border-right: 100px solid transparent;
}

#triangle-bottomright {
    width: 0;
    height: 0;
    border-bottom: 100px solid red;
    border-left: 100px solid transparent;
}

<!-- 外框 -->
<div class="mod-container">
    <!-- 小三角部分 -->
    <div class="mod-triangle">
        <div class="t-border"></div>
        <div class="t-inset"></div>
    </div>
</div>

/*外框容器*/
.mod-container {
    width:100px;
    height:60px;
    border:1px solid #000;
    margin:20px;
    background:#fff;
}
/*小三角部分*/
.mod-triangle {
    display:block;
    position:relative;
    left:-23px;
    top:18px;
    z-index:20;
}
.mod-triangle .t-border,
.mod-triangle .t-inset{
    left:0px;
    top:0px;
    width:0;
    height:0;
    font-size:0;
    overflow:hidden;
    position:absolute;
    border-width:12px;
    /*可在此处更改小三角方向：上-右-下-左（solid的位置）*/
    border-style:dashed solid dashed dashed;
}
/*小三角的边框，如果不需要，可将颜色值改变*/
.mod-triangle .t-border{
    border-color:transparent #000 transparent transparent;
    left:-1px;
}
.mod-triangle .t-inset{
    border-color: transparent #fff transparent transparent;
}