<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<title>Document</title>
<script></script>
<style>
.flex-container {
display: flex;
color: white;
}
.flex-container .flex-item {
background-color: red;
width: 100px;
}
.sel {
background-color: black;
}
</style>
</head>
<body style="width: 100%">
<div class="flex-container">
<div class="flex-item sel">
</div>
<div class="flex-item">
</div>
<div class="flex-item">
</div>
</div>
</body>
</html>
sel不会生效，因为css先按权重排序，权重大的样式优先使用，权重一样的才按先后顺序
.flex-container .flex-item 有两个选择器，权重是两个相加，所以权重大
改成：
.flex-item {
background-color: red;
width: 100px;
}
.sel {
background-color: black;
}
或者：
.flex-container .flex-item {
background-color: red;
width: 100px;
}
.flex-item.sel {
background-color: black;
}
都可以使sel可以生效
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