如何用css写三角形？

.increace {
  width: 0%;
  height: 0%;
  border-width: 3px;
  border-color: transparent transparent red transparent;
  border-style: solid;
  margin: 0 0 3px 5px;
}
.decreace {
  width: 0%;
  height: 0%;
  border-width: 3px;
  border-color: green transparent transparent transparent;
  border-style: solid;
  margin: 3px 0 0 5px;
}