垂直居中绝对div，没有定义的高度
这个问题在这里已经有了答案：
您可以将.child放在top: 50%; 并通过transform: translateY(-50%); ，这将以你的元素为中心。 这会将元素从父元素的顶部放置50％，并将其移至子元素高度的50％。
.parrent{
position: relative;
}
.child {
background: red;
width: 50px;
height: 50px;
position: absolute;
top: 50%;
transform: translateY(-50%);
}
在这个小提琴上看到工作解决方案
你可以使用这个片段：
https://jsfiddle.net/ocbzdkpd/1/
基本上你需要：
top: 50%;
transform:translateY(-50%);
因为箱子的大小没有关系。
Flexbox可以帮助你。
.parent{
display:flex;
align-items:center; // vertical alignment
}
.child {
background: red;
width: 50px;
height: 50px;
position: absolute;
}<div class="parent">
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<div class="child"></div>
</div>