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>