效果图
0.5px边框
0.5px边框，可以用伪元素实现，before、after都可以。移动端H5页面用的比较多。
圆角渐变色边框+圆角渐变色背景
实现外部圆角边框，内部圆角渐变色背景，是用父div + 子div实现的，其中父div和子div用不同的背景颜色，父div的padding大小，就是整体的边框高度。
如果内部div不需要圆角，可以参考这篇文章：CSS 巧妙实现带圆角的渐变边框
代码
<!DOCTYPE html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1.0,maximum-scale=1.0,user-scalable=0"/><style>.half-border{position: relative;width: 200px;height: 100px;line-height: 100px;text-align: center;}.half-border::after{content: "";position: absolute;top: 0;left: 0;box-sizing: border-box;width: 200%;height: 200%;border-width: 1px;border-style: solid;border-color: red;border-radius: 10px;transform-origin: 0 0;transform: scale(0.5);/* z-index: 0; */}.border{width: 200px;height: 100px;margin-left: 20px;border: 1px solid red;border-radius: 10px;line-height: 100px;text-align: center;}.out-border-box{width: 420px;height: 50px;padding: 6px;margin-top: 20px;box-sizing: border-box;line-height: 100px;border-radius: 24px;background-image: linear-gradient(to left, rgba(0,199,119,0) 0%, rgba(0,199,119,0.2) 100%);}.inner-content-box{width: 100%;height: 100%;color: #333333;display: flex;justify-content: flex-start;align-items: center;padding-left: 20px;border-radius: 24px;background: #fff;background-image: linear-gradient(to left, rgba(2,220,136,0) 0%, rgba(168,255,223,0.15) 100%);}.half-border-bottom {width: 200px;height: 100px;margin-top: 20px;position: relative;}.half-border-bottom:before {content: "";position: absolute;box-sizing: border-box;width: 200%;bottom: 0;border-bottom: 1px solid red;-webkit-transform-origin: 0 0;-webkit-transform: scale(0.5, 0.5);transform-origin: 0 0;transform: scale(0.5, 0.5);}.border-bottom {width: 200px;height: 100px;margin-top: 20px;border-bottom: 1px solid red;margin-left: 20px;}</style></head><body><div style="display: flex;"><div class="half-border">0.5px边框</div><div class="border">1px边框</div></div><div style="display: flex;"><div class="half-border-bottom" >0.5px边框</div><div class="border-bottom" >1px边框</div></div><div class="out-border-box"><div class="inner-content-box">渐变边框、渐变背景</div></div></body>
