margin-top -50%

// html 代码  
<div class="parent">
  <div class="child">
  </div>
</div>
// css 代码
<style>
.parent{
  height: 300px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  width: 300px;
  height: 100px;
  position: absolute;
  top: 50%;
  left: 50%;
  margin-left: -150px;
  margin-top: -50px;
}
</style>

translate -50%

// html 代码
<div class="parent">
  <div class="child">
    test
  </div>
</div>
// css 代码
<style>
.parent{
  height: 300px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  width: 300px;
  height: 100px;
  position: absolute;
  top: 50%;
  left: 50%;
  margin-left: -150px;
  margin-top: -50px;
}
</style>

absolute margin auto

// HTML 代码
<div class="parent">
  <div class="child">
  </div>
</div>
// CSS 代码
<style>
.parent{
  height: 300px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  position: absolute;
  width: 100px;
  height: 100px;
  margin: auto;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}
</style>

flex

// HTML 代码
<div class="parent">
  <div class="child">
  </div>
</div>
// CSS 代码
<style>
.parent{
  height: 300px;
  border: 1px solid red;
  display: flex;
  justify-content: center;
  align-items: center;
}
.child{
  width: 100px;
  height: 100px;
  border: 1px solid green;
  width: 300px;
}
</style>