.parent {
width: 500px; /* 为了显示方便，与本文知识点无关 */
background-color: teal; /* 为了显示方便，与本文知识点无关 */
height: 300px;
position: relative;
}
.child {
background-color: tomato;
color: #fff;
width: 100px;
position: absolute;
top: 50%;
transform: translateY(-50%);
}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>块级元素垂直居中</title>
</head>
<body>
<div class="parent">
<div class="child">
我是块级元素，我要垂直居中
</div>
</div>
</body>
</html>