怎么利用CSS3绘制三角形

最近三角形挺火，很多地方都能碰到，如网页，微信，或者QQ空间的时间轴等地方都能看到，而且这些并不是图片插入进去的，那就需要用CSS来做了

<p class="bbb">111111111111</p>
<br/>
<div class="triangle-up">     <!--向上的三角--> </div>
<br/>
<div class="triangle-down">    <!--向下的三角--> </div>
<div class="triangle-left">    <!--向左的三角--> </div>
<div class="triangle-right">    <!--向右的三角--> </div>

 1 <style>
 2     .bbb {
 3         width: 300px;
 4         height: 200px;
 5         background: forestgreen;
 6         margin: auto;
 7         padding: 10px;
 8         color: #FFFFFF;
 9         position: relative;
10
11     }
12     .bbb:before {
13         content: ‘‘;
14         display: block;
15         position: absolute;
16         left: 20px;
17         top: -10px;
18         width: 0;
19         height: 0;
20         border: 10px solid transparent;
21         border-top: 0 none;
22         border-bottom-color: forestgreen;
23     }
24     .triangle-up {
25         width: 0;
26         height: 0;
27         border-left: 30px solid transparent;
28         border-right: 30px solid transparent;
29         border-bottom: 30px solid green;
30     }
31     .triangle-down {
32         width: 0;
33         height: 0;
34         border-left: 20px solid transparent;
35         border-right: 20px solid transparent;
36         border-top: 20px solid blue;
37     }
38     .triangle-left {
39         width: 0;
40         height: 0;
41         border-top: 20px solid transparent;
42         border-bottom: 20px solid transparent;
43         border-right: 20px solid red;
44     }
45     .triangle-right {
46         width: 0;
47         height: 0;
48         border-top: 20px solid transparent;
49         border-bottom: 20px solid transparent;
50         border-left: 20px solid green;
51     }
52     .triangle-up, .triangle-down, .triangle-left, .triangle-right{  display: block; margin: auto;}
53     </style>

效果图：