<!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"> 1 </div> <div class="flex-item"> 2 </div> <div class="flex-item"> 3 </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
