box-sizing属性

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            box-sizing: border-box;
            width: 200px;
            height: 200px;
            border: 10px solid #000;
            padding: 10px;
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>

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CSS

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