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;
}

<div class="box">
</div>

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;
}

<div class="drop">
  <div class="box">
  </div>
</div>

Answer 2

In my opinion you would either have to work with borders (as you did) or SVGs. For the shadow it does not work with borders and regular css-shadows, so you want to apply drop-shadow as a filter (or you can use a SVG for the shadow).

This is what I came up with

.triangle_left {
  width: 600px;
  height: 600px;
  position: relative;
  margin: 20px auto;
  border: 2px solid #cccccc;
  /* box-shadow: 0px 0px 8px 1px darkgrey; */
  border-radius: 20px;
}

.triangle_left:before {
  content: '';
  width: 0;
  height: 0;
  border-top: 30px solid transparent;
  border-left: 30px solid white;
  border-bottom: 30px solid transparent;
  filter: drop-shadow(4px 0px 0px #ccc);
  position: absolute;
  left: -6px;
  top: 50%;
}

.triangle_left:after {
  content: '';
  width: 0;
  height: 0;
  border-top: 30px solid transparent;
  border-right: 30px solid white;
  border-bottom: 30px solid transparent;
  filter: drop-shadow(-4px 0px 0px #ccc);
  position: absolute;
  right: -6px;
  top: 50%;
}

<div class="triangle_left">

</div>