Q2：页面元素的水平垂直居中

.parent {
  width: 300px;
  height: 300px;
  display: flex;
  justify-content: center;
  align-item: center;
  background: yellow;
}
.child {
  width: 100px;
  height: 100px;
  background: red;
}

.parent {
  position: relative;
}

.child {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
}

.parent {
  position: relative;
}

.child {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  margin: auto;
  width: 100px;
  height: 100px;
}

.parent {
  position: relative;
}

.child {
  position: absolute;
  top: 50%;
  left: 50%;
  margin-top: -50px;
  margin-left: -50px;
  width: 100px;
  height: 100px;
}

.parent {
  display: table-cell;
  vertical-align: middle;
  text-align: center;
}

.child {
  display: inline-block;
}

.parent {
  display: grid;
}

.child {
  align-self: center;
  justify-self: center;
}