CSS 元素居中

1. flex

.box {
  display: flex;
  justify-content: center;
  align-items: center;
}

2. flex 结合 margin:auto

.box {
  display: flex;
}
.box .item {
  margin: auto;
}

3. 子元素 absolute + transform

.box {
  position: relative;
}
.box .item {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

4. 子元素 absolute 撑满 + margin auto

.box .item {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  margin: auto;
}