//已知宽高的1
.wrap {
  width: 300px;
  height: 300px;
  background-color: red;
  position: absolute;//或者/fixed
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  margin: auto;
}

//已知宽高的2.1
.wrap{
    width: 300px;
    height: 300px;
    background-color: red;
    position: absolute;
    top: 50%;
    margin-top: -150px;
    left: 50%;
    margin-left: -150px;
}

//未知宽高的，靠内容撑的2.2
.wrap{
    background-color: red;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

//父元素flex布局的，3
body{
    display: flex;
    align-items: center;
    justify-content: center;
}

.wrap {
  width: 300px;
  height: 300px;
  background-color: red;
}