方式一：flex 布局
<style>
.box {
200px;
height: 200px;
border: 1px solid blue;
position: relative;
display: flex;
align-items: center;
justify-content: center;
}
.item {
background: red;
}
</style>
<div class="box">
<div class="item">
<h1 >宽高不定</h1>
</div>
</div>
方式二： Position + transform
<style>
.box {
200px;
height: 200px;
border: 1px solid blue;
position: relative;
}
.item {
background: red;
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
}
</style>
<div class="box">
<div class="item">
<h1>宽高不定</h1>
</div>
</div>
方式三：position + margin
<style>
.box {
200px;
height: 200px;
border: 1px solid blue;
position: relative;
}
.item {
50%;
height: 50%;
background: red;
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
}
</style>
<div class="box">
<div class="item">
<h1>宽高不定</h1>
</div>
</div>
方式四： Grid布局
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>三十课 - 利用grid布局实现元素水平垂直居中示例</title>
<style>
.parent {
height: 140px;
display: grid;
align-items:center;
border: 2px dashed #f69c55;
}
.child {
margin:auto;
padding: 20px;
10rem;
color: #fff;
background: black;
}
</style>
</head>
<body>
<div class="parent">
<div class="child">好的程序员能写出人能读懂的代码。</div>
</div>
</body>
</html>