CSS 实现三角形的2种方法

一、利用border
1、元素本身的长宽为0
2、将不需要的部分通过 border-color 来设置隐藏

//html
<div id="first"></div>
<div id="second"></div>
<div id="third"></div>
//css
#first {
  width: 20px;
  height: 20px;
  border-width: 10px;
  border-style: solid;
  border-color: red green blue brown;
}
 
#second {
  width: 0;
  height: 0;
  border-width: 10px;
  border-style: solid;
  border-color: red green blue brown;
}
 
#third {
  width: 0;
  height: 0;
  border-width: 10px;
  border-style: solid;
  border-color: red transparent transparent transparent;
}

效果：

Paste_Image.png

二、利用transform旋转 45 度实现三角形
我们来实现一个带小三角的提示框

//html
<div class="message-box">  
  <span>我是利用 css transfrom 实现的</span>  
  <div class="triangle-css3 transform ie-transform-filter"></div>
</div>
//css
.message-box {
    position:relative;
    width:240px;
    height:60px;
    line-height:60px;
    background:#E9FBE4;
    box-shadow:1px 2px 3px #E9FBE4;
    border:1px solid #C9E9C0;
    border-radius:4px;
    text-align:center;
    color:#0C7823;
}
.triangle-css3 {
    position:absolute;
    bottom:-8px;
    bottom:-6px;
    left:30px;
    overflow:hidden;
    width:13px;
    height:13px;
    background:#E9FBE4;
    border-bottom:1px solid #C9E9C0;
    border-right:1px solid #C9E9C0;
}
.transform {
    -webkit-transform:rotate(45deg);
    -moz-transform:rotate(45deg);
    -o-transform:rotate(45deg);
    transform:rotate(45deg);
}
/*ie7-9*/
.ie-transform-filter {
    -ms-filter: "progid:DXImageTransform.Microsoft.Matrix(
        M11=0.7071067811865475,
        M12=-0.7071067811865477,
        M21=0.7071067811865477,
        M22=0.7071067811865475,
    SizingMethod='auto expand')";
    filter: progid:DXImageTransform.Microsoft.Matrix(
        M11=0.7071067811865475,
        M12=-0.7071067811865477,
        M21=0.7071067811865477,
        M22=0.7071067811865475,
    SizingMethod='auto expand');
}

Paste_Image.png

另：利用transform实现一个四边形
主要是借助了transform: skew(...)

//html
<div class="city">上海</div>
//css
.city {
  display: inline-block;
  padding: 5px 20px;
  border: 1px solid #44a5fc;
  color: #333;
  transform: skew(-20deg);
}

Paste_Image.png

最后，附上jsfiddle示例
https://jsfiddle.net/fozero/d2hbda2v/

参考：
http://m.blog.csdn.net/article/details?id=51490107
http://web.jobbole.com/83248/