细边和直角
2021/11/23    
box-shadow:0 0 2px #00ffff inset
background:linear-gradient(#00ffff, #00ffff) left top,
        linear-gradient(#00ffff, #00ffff) left top,
        linear-gradient(#00ffff, #00ffff) right bottom,
        linear-gradient(#00ffff, #00ffff) right bottom
background-size:1px 6px, 6px 1px

四个边和投影

https://blog.csdn.net/weixin_53741561/article/details/127192278