How to create a corset like effect with CSS?

Question

I want to create a effect as below with CSS, initially, I was thinking to merge two rectangle with rounded corner, but I can\'t use that way since it is a sharp angle, not round

Answer 1

An idea is to build the shape using gradient and rely on drop-shadow filter

.box {
  margin-top:50px;
  border-radius:10px;
  width:200px;
  height:200px;
  background:
  /*the middle line */
    repeating-linear-gradient(to right,gold 0px,gold 5px,transparent 5px,transparent 10px) center/calc(100% - 40px) 2px,
    /*The background*/
    linear-gradient(to bottom right,#fff 49%,transparent 50%) 100% calc(50% - 10px) / 20px 20px,
    linear-gradient(to top    right,#fff 49%,transparent 50%) 100% calc(50% + 10px) / 20px 20px,
    linear-gradient(to bottom left ,#fff 49%,transparent 50%) 0    calc(50% - 10px) / 20px 20px,
    linear-gradient(to top    left ,#fff 49%,transparent 50%) 0    calc(50% + 10px) / 20px 20px,
    
    linear-gradient(#fff,#fff) top    right / 20px calc(50% - 20px),
    linear-gradient(#fff,#fff) bottom right / 20px calc(50% - 20px),
    
    linear-gradient(#fff,#fff) top    left / 20px calc(50% - 20px),
    linear-gradient(#fff,#fff) bottom left / 20px calc(50% - 20px),
    
    linear-gradient(#fff,#fff) center/calc(100% - 40px) 100%;
   background-repeat:no-repeat;
   filter:drop-shadow(0 0 5px #000); 
}

body {
  background:pink;
}

You can also do it with clip-path but you need an extra wrapper where you apply the drop-shadow filter

.box {
  margin-top: 50px;
  border-radius: 10px;
  width: 200px;
  height: 200px;
  background: 
   repeating-linear-gradient(to right, gold 0px, gold 5px, transparent 5px, transparent 10px) center/100% 2px no-repeat,
   #fff;
  clip-path:polygon(
  0 0,100% 0,
  100% calc(50% - 20px),calc(100% - 20px) 50%,100% calc(50% + 20px),
  100% 100%, 0 100%,
  0 calc(50% + 20px),20px 50%,0 calc(50% - 20px)
  );
}

.drop {
  filter: drop-shadow(0 0 5px #000);
}

body {
  background: pink;
}