css：比1px更細的線

#demo{position: relative;}
#demo:after{content:'';border:1px solid #999;height:200%;width:200%;-webkit-transform: scale(0.5);transform:0.5;transform-origin: 0 0;-webkit-transform: 0 0;position: absolute;top:0;left:0;}

<div class="demo">我有0.5px的border</div>