element {
    width: 100px;    // width === height
    height: 100px;
    border-radius: 50%;
  }
          
        

              
  element {
    width: 0;    // width === height === 0
    height: 0;
    border-bottom: 25px solid green;
    border-left: 25px solid yellow;
    border-right: 25px solid red;
    border-top: 25px solid blue;
  }              
                
              

          
  element {
    width: 0;    // width === height === 0
    height: 0;
    border-bottom: 25px solid green;
    border-left: 25px solid transparent;  //transparent
    border-right: 25px solid transparent;
    border-top: 25px solid transparent;
  }  
                  
        

          
    //transparent
    border-left: 25px solid yellow;
    //transparent
          
        

            
      //transparent
      border-top: 25px solid blue;
      //transparent
            
          

              
        //transparent
        border-top: 25px solid red;
        //transparent