首先看效果，将显示比例调到200%，效果更明细
上代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
<style>
.outer {
position: relative;
width: 300px;
height: 300px;
background-color: #bed4e6e7;
border: 1px solid #1500ffbe;
}
.inner {
position: absolute;
left: 0px;
top: 0px;
width: 300px;
height: 50px;
border: 1px solid #1500ffbe;
}
</style>
</head>
<body>
<div>
<div class="outer">
<div class="inner"></div>
</div>
</div>
</body>
<script></script>
</html>
然后我们把子元素的边框宽度调大，如下
很明显，是因为定位会计算边框的。因此定位向左上移动父元素边框的宽度就好了