父容器设置 position: relative;

0.5px 下边距

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&::after {
    content: "";
    position: absolute;
    bottom: 0;
    left: 0;
    right: 0;
    border-bottom: 1px solid #dadada;
    transform: scaleY(.5);
    transform-origin: left bottom;
}

0.5px 上边距

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&::before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    border-bottom: 1px solid #dadada;
    transform: scaleY(.5);
    transform-origin: left top;
}

0.5px圆角边框

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&::after {
    content: "";
    box-sizing: border-box;
    display: block;
    position: absolute;
    width: 200%;
    height: 200%;
    top: 0;
    left: 0;
    right: 0;
    border: 1px solid rgba(229, 71, 67, .3);
    border-radius: 50/75rem;
    transform: scale(.5);
    transform-origin: left top;
}

注意 transform 的起点，上边距要用左上角，下边距用左下角，且box-sizing要设置为border-box，否则伪元素上下左右各会多1px。