如何用CSS画三角形

.border-all-color {
 margin: 20px auto;
 width: 0;
 height: 0;
 border-top: 40px solid coral;
 border-right: 40px solid lightblue;
 border-bottom: 40px solid lightgreen;
 border-left: 40px solid mediumpurple;
}

.border-left-color,
.border-top-color,
.border-right-color,
.border-bottom-color{
 width: 0;
 height: 0;
 margin: 10px auto;
 border: 20px solid transparent;
}

.border-left-color{
 border-left-color: #9370DB;
}
.border-top-color{
 border-top-color: #FF7F50;
}
.border-right-color{
 border-right-color: #ADD8E6;
}
.border-bottom-color{
 border-top-color: #90EE90;
 }