css直接写出小三角

　　　　　　　　　/*箭头向上*/
.arrow-up {
width:0;
height:0;
border-left:30px solid transparent;
border-right:30px solid transparent;
border-bottom:30px solid #000;
}

/*箭头向下*/
.arrow-down {
width:0;
height:0;
border-left:30px solid transparent;
border-right:30px solid transparent;
border-top:30px solid #000;
}

/*箭头向左*/
.arrow-left {
width:0;
height:0;
border-top:30px solid transparent;
border-bottom:30px solid transparent;
border-right:30px solid #000;
}

/*箭头向右*/
.arrow-right {
width:0;
height:0;
border-top:30px solid transparent;
border-bottom: 30px solid transparent;
border-left: 30px solid #000;
}

/*:after 伪元素在元素之后添加内容实现小三角*/
.box{ width:300px; height:300px; background:#838383; position:relative;}
.box:after{
position:absolute;
right:-20px;
top:10px;
display:block;
content:"";
width:0;
height:0;
border-top:20px solid transparent;
border-bottom: 20px solid transparent;
border-left: 20px solid #838383;
}