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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        html,body{
            margin: 0;
            padding: 0;
            height: 100%;
            width: 100%;
        }
        .box{
            display: flex;
            flex-flow: wrap;
            width: 500px;
        }
        .box div{
            flex-grow:1;
            width: 49%;
            height: 50px;
            border: sienna solid 1px;
        }
    </style>
</head>
<body>
<div class="box">
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
    <div></div>
</div>
</body>
</html>

css flex换行填满父容器

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源码