实现效果图如下：

css代码：

知识点：resize: horizontal; 文档地址

 <style>
        .image-slider {
            position: relative;
            display: inline-block;
            width: 500px;
            height: 300px;
        }

        .image-slider>div {
            position: absolute;
            top: 0;
            bottom: 0;
            left: 0;
            width: 50%;
            max-width: 100%;
            overflow: hidden;
            resize: horizontal;
        }

        .image-slider>div:before {
            content: '';
            position: absolute;
            right: 0;
            bottom: 0;
            width: 12px;
            height: 12px;
            padding: 5px;
            background: linear-gradient(-45deg, white 50%, transparent 0);
            background-clip: content-box;
            cursor: ew-resize;
            -webkit-filter: drop-shadow(0 0 2px black);
            filter: drop-shadow(0 0 2px black);
        }

        .image-slider img {
            display: block;
            user-select: none;
            width: 100%;
            height: 100%;
        }
    </style>

html结构：

<div class="image-slider">
       <div><img src="./imgs/before.png" alt="before" /></div>
       <img src="./imgs/after.png" alt="after"/>
 </div>