居中的多种实现

方法1

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  display: grid;
  align-items: center;
  justify-items: center;
}
p {
  background: red;
}

方法2（掌握）

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  display: grid;
  align-items: center;
  justify-content: center;
}
p {
  background: red;
}

方法3（掌握）

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  display: grid;
}
p {
  background: red;
  margin: auto;
}

方法4（掌握）

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  display: flex;
  align-items: center;
  justify-content: center;
}
p {
  background: red;
}

方法5（掌握）

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  display: flex;
}
p {
  background: red;
  margin: auto;
}

方法6

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  display: table-cell;
  vertical-align: middle;
  text-align: center;
}
p {
  background: red;
  display: inline-block;
}

方法7（掌握）

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  position: relative;
}
p {
  position: absolute;
  left: 50%;
  top: 50%;
  transfrom: translate(-50%, -50%);
  margin: 0;
}

方法8

<div class="box">
  <p>hello</p>
</div>

.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  text-align: center;
}
.box::after {
  content: '';
  line-height: 200px;
}
p {
  display: inline-block;
}

方法9(废弃)

<div class="box">
  <p>hello</p>
</div>

/* method 9*/
.box {
  width: 200px;
  height: 200px;
  border: 1px solid;
  position: relative;
}
p {
  background: red;
/*必须要设置宽高*/
  width: 100px;
  height: 40px;
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  margin: auto;
}

总结
• 推荐使用flex居中的方式