how to make a square using css code example

Example 1: css circle

#circle {
      width: 100px;
      height: 100px;
      background: red;
      border-radius: 50%
    }

Example 2: css triangle

.arrow-up {
  width: 0; 
  height: 0; 
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  
  border-bottom: 5px solid black;
}

.arrow-down {
  width: 0; 
  height: 0; 
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
  
  border-top: 20px solid #f00;
}

.arrow-right {
  width: 0; 
  height: 0; 
  border-top: 60px solid transparent;
  border-bottom: 60px solid transparent;
  
  border-left: 60px solid green;
}

.arrow-left {
  width: 0; 
  height: 0; 
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent; 
  
  border-right:10px solid blue; 
}