单边框0.5px，可以用以下方式：

方式一：border + border-image + 线性渐变linear-gradient

<div class="border">0.5像素边框</div>
<style>
    .border {
        width: 200px;
        height: 200px;
        margin: 0 auto;
        border-bottom: 1px solid transparent;
        border-image: linear-gradient(to bottom,transparent 50%, red 50%) 0 0 100%/1px 0;
    }
</style>

方式二：定位 + 伪元素 + background + 线性渐变linear-gradient

<div class="border">0.5像素边框</div>
<style>
    .border {
        width: 200px;
        height: 200px;
        margin: 0 auto;
        position: relative;
    }
    .border::before {
        content: " ";
        position: absolute;
        left: 0;
        bottom: 0;
        width: 100%;
        height: 1px;
        background-image: linear-gradient(to bottom, transparent 50%, red 50%);
    }
</style>

方法三：定位 + 伪元素 + transfrom缩放（scale）

<div class="border">0.5像素边框</div>
<style>
    .border {
        width: 200px;
        height: 200px;
        margin: 0 auto;
        position: relative;
    }
    .border::after {
        content: " ";
        position: absolute;
        left: 0;
        bottom: 0;
        width: 100%;
        height: 1px;
        background: red;
        transform: scaleY(0.5);
    }
</style>

对于需要四边0.5像素边框，可以用以下方式：

方式：定位 + 伪元素 + transfrom缩放（scale）

<div class="border">0.5像素边框~~~~</div>
<style>
    .border {
        width: 200px;
        height: 200px;
        margin: 0 auto;
        position: relative;
    }
    .border::before {
        content: " ";
        position: absolute;
        top: 0;
        left: 0;
        width: 200%;
        height: 200%;
        border: 1px solid red;
        transform-origin: 0 0;
        transform: scale(0.5);
    }
</style>

链接: https://fly63.com/article/detial/5825