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css父元素背景覆盖其子元素背景
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<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style> .test1{
width: 100px;
height: 100px;
border: 1px solid green;
background: green;
position: absolute;
z-index: auto;
} .test2{
width: 100px;
height: 100px;
border: 1px solid black;
background: black;
position: relative;
left: 20px;
top: -20px;
z-index: -1;
}
</style>
</head>
<body>
<div class="test1">
<div class="test2"></div> </div> </body>
</html>
父元素不设置z-index 的值时，默认是auto。此时在设置子元素的值为负数
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