適用場景：父子寬高都可未知(比較推薦這種方式，簡單，而且目前兼容性也不錯。)
<html>
<head>
<style>
.parent {
width: 100%;
height: 100px;
background: cyan;
display: flex;
justify-content: center;
align-items: center;
}
.son {
width: 20%;
height: 20%;
background: pink;
}
</style>
</head>
<body>
<div class='parent'>
<div class='son'></div>
</div>
</body>
</html>適用場景：父元素寬高已知未知都行，但是首先得有寬高。其次子元素的寬高必須已知，因爲需要設置子元素的負margin. (也可以將負margin設置成translate來做位移實現)
<html>
<head>
<style>
.parent {
position: relative;
width: 200px;
height: 200px;
background: pink;
}
.son {
position: absolute;
left: 50%;
top: 50%;
margin-left: -25px;
margin-top: -25px;
width: 50px;
height: 50px;
background: yellow;
}
</style>
</head>
<body>
<div class='parent'>
<div class='son'></div>
</div>
</body>
</html>適用場景：父子元素寬高都未知的情況(該方式不需要明確知道子元素寬高，子元素寬高可用百分比，對於子元素寬高不確定需要居中的情況比較適合。)
<html>
<head>
<style>
.parent {
position: relative;
width: 200px;
height: 200px;
background: cyan;
}
.son {
position: absolute;
left: 0;
top: 0;
bottom: 0;
right: 0;
margin: auto;
width: 10%;
height: 10%;
background: yellow;
}
</style>
</head>
<body>
<div class='parent'>
<div class='son'></div>
</div>
</body>
</html>適用場景：父子元素寬高未知，兼容性不大好
<html>
<head>
<style>
.parent {
display: grid;
}
.son {
jusitify-self: center;
align-self: center;
}
</style>
</head>
<body>
<div class='parent'>
<div class='son'></div>
</div>
</body>
</html>適用場景: 父元素大小已知（非百分比高度），子元素大小未知，但子元素須爲行內塊元素，較好的兼容性
<html>
<head>
<style>
.parent {
display: table-cell;
vertical-align: middle;
text-align: center;
width: 100vw;
height: 90vh;
background-color: yellowgreen;
}
.son {
display: inline-block;
width: 200px;
height: 200px;
background-color: Indigo;
}
</style>
</head>
<body>
<div class='parent'>
<div class='son'></div>
</div>
</body>
</html>適用場景：父子寬高都可未知，子元素需爲行內塊元素(這種方式其實就是使用僞元素的高度爲100%，子元素和僞元素都設置 vertical-align: middle實現垂直居中的效果)
<html>
<head>
<style>
.parent {
height: 100vh;
width: 100vw;
text-align: center;
background: #c0c0c0;
}
.parent:before {
content: "\200B";
display: inline-block;
height: 100%;
vertical-align: middle;
}
.son {
display: inline-block;
vertical-align: middle;
width: 200px;
height: 200px;
padding: 10px 15px;
background: #f5f5f5;
}
</style>
</head>
<body>
<div class="parent">
<div class="son"></div>
</div>
</body>
</html>