body {
  font-family: Arial, sans-serif;
  background-color: #111;
  color: #fff;
  margin: 0;
  padding: 0;
}

 #game {
     width: 500px;
     height: 200px;
     border: 1px solid black;
     margin: auto;
   }
   
    #character {
     width: 20px;
     height: 50px;
     background-color: red;
     position: relative;
     top: 150px; //game height - character height (200 - 50)
 }
 
  @keyframes jump {
     0%{top: 150px;}
     30%{top: 100px;}
     70%{top: 100px;}
     100%{top: 150px;}
 }
 
   #block {
      width: 20px;
      height: 20px;
      background-color: blue;
      position: relative;
      top: 130px; //game height - character height - block height (200 - 50 - 20)
      left: 480px; //game width - block width (500 - 20)
     animation: block 1s infinite linear;
  }
  
   @keyframes block {
     0%{left: 500px} 
     100%{left: -20px}
 }