style.css

@import url('https://fonts.googleapis.com/css2?family=Roboto&display=swap');
* {
  box-sizing:border-box;
}
body{
  margin:0;
  font-family:'Roboto', Arial;
}
.container{
  display:flex;
  flex-direction:column;
  align-items:center;
  justify-content:center;
  height:100vh;
  width:100%;
  text-align:center;
  overflow:hidden;
}
.playground{
  display:grid;
  grid-template-columns:100px 100px 100px;
  justify-content:center;
  align-items:center;
  align-content:center;
  width:350px;
  height:350px;
  margin:0 auto;
}
.box{
  border:2px solid #000;
  width:100px;
  height:100px;
  cursor:pointer;
  display:flex;
  justify-content:center;
  align-items:center;
  font-size:76px;
  font-weight:600;
}

.box:first-child, .box:nth-child(2),.box:nth-child(3){
  border-top:none;
}
.box:nth-child(3n + 1){
  border-left:none;
}
.box:nth-child(3n + 3){
  border-right:none;
}
.box:last-child, .box:nth-child(8),.box:nth-child(7){
  border-bottom:none;
}