三角形

.triangle{
    width: 0;
    height: 0;
    border-width: 10px;
    border-style: solid;
    border-color: transparent transparent transparent red; 
}

高自适应的正方形

.square {
  width: 10%;
  height: 10vw;
  background: tomato;
}
.square {
  width: 20%;
  height: 0;
  padding-top: 20%;
  background: orange;
}
.square {
  width: 30%;
  overflow: hidden;
  background: yellow;
}
.square::after {
  content: "";
  display: block;
  margin-top: 100%;
}