纯CSS实现三角形、扇形

html:

<div class="triangle"></div>
<div class="arc"></div>

.triangle {
    width: 0;
    height: 0;
    border-width:50px;
    border-style: solid;
    border-color: #f00 transparent transparent;
}
.arc {
    //triangle的代码 + border-radius: 50%
    width: 0;
    height: 0;
    border-width:50px;
    border-style: solid;
    border-color: #f00 transparent transparent;
}