HTML

<div class="parent answer-1">
  <div></div>
</div>

CSS

.parent {
  width: 800px;
  height: 800px;
  border: 1px solid red;
}
.parent div {
  width: 200px;
  height: 200px;
  border: 1px solid red;
}

/*方法1:dispaly:table-cell*/
.answer-1 {
  display: table-cell;
  vertical-align: middle;
}
.answer-1 div {
  margin: auto;
}
/*方法2：绝对定位(高度必须固定)*/
.answer-2 {
  position: relative;
}
.answer-2 div {
  position: absolute;
  /*垂直居中*/
  top: 0;
  bottom: 0;
  margin-top: auto;
  margin-bottom: auto;
  /*水平居中*/
  left: 0;
  right: 0;
  margin-left: auto;
  margin-right: auto;
}
/*方法3：弹性布局(高度可不固定)*/
.answer-3 {
  display: flex;
  /*垂直居中*/
  align-items: center;
  /*水平居中*/
  justify-content: center;
}
/*方法4：绝对定位 + 2d转换(高度可不固定)*/
.answer-4 {
  position: relative;
}
.answer-4 div {
  position: absolute;
  /*垂直居中*/
  top: 50%;
  /*水平居中*/
  left: 50%;
  transform: translate(-50%,-50%);
}