小三角

箭头朝向那个方向就不用写他的border,在反方向设置颜色 其他方向上的透明度为0

/*箭头向上*/
.arrow-up {
    width:0; 
    height:0; 
    border-left:30px solid transparent;
    border-right:30px solid transparent;
    border-bottom:30px solid #fff;
}
    
 /*箭头向下*/
.arrow-down {
    width:0; 
    height:0; 
    border-left:20px solid transparent;
    border-right:20px solid transparent;
    border-top:20px solid #0066cc;
}
    
 /*箭头向左*/
.arrow-left {
    width:0; 
    height:0; 
    border-top:30px solid transparent;
    border-bottom:30px solid transparent; 
    border-right:30px solid yellow; 
}
   
/*箭头向右*/
.arrow-right {
    width:0; 
    height:0; 
    border-top:50px solid transparent;
    border-bottom: 50px solid transparent;
    border-left: 50px solid green;
}