很常见的需求

解法1

利用object-fit:contain方法处理,这个方法使用很简单 但是不兼容ie 在ie下默认填充100% 很不友好

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<!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">
    <title>Document</title>
    <style>
        .box1,
        .box2 {
            width: 300px;
            height: 300px;
            border: 1px red solid;
            margin: 10px auto;
        }

        img {
            width: 100%;
            height: 100%;
            object-fit: contain;
        }
    </style>
</head>

<body>
    <div class="box1"><img src="./1.jpg" alt=""></div>
    <div class="box2"><img src="./2.jpg" alt=""></div>
</body>

</html>

解法2

更推荐的用法 对ie兼容好 样式不会发生变化

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<!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">
    <title>Document</title>
    <style>
        .box1 {
            background-image: url(./1.jpg);
        }

        .box2 {
            background-image: url(./2.jpg);
        }

        .box1,
        .box2 {
            width: 300px;
            height: 300px;
            border: 1px red solid;
            background-size: contain;
            background-repeat: no-repeat;
            background-position: center center;
            margin: 10px auto;
        }
    </style>
</head>

<body>
    <div class="box1"></div>
    <div class="box2"></div>
</body>

</html>

示例

解法1示例
解法2示例