developed QR code component using css and html.

Looks good!

One thing I would mention is to add some padding to your .box class :

.box {
  background-color: white;
  border: 2px ;
  border-radius: 20px;
  margin: auto;
  width: 320px;
  height: 473px;
  padding : 10px;
}

And maybe just the background color of the body :

body{ background-color: hsl(212, 45%, 89%); }

But with that said its a great attempt!