#a{position:fixed;
 width:50px;
    height:50px;
    border-radius:100%;
    background-color:#F00;
    animation:10s aa infinite;    }
@keyframes aa {
   0%{left:0px;
     top:0px;}
25%{left:100% ;
margin-left:-50px ;
top:0px;
margin-top:0px ;
        }
50%{top:100%;
        margin-top:-50px;
left:100%;
        margin-left:-50px;
}
75%{left:0px;
        margin-left:0px;
top:100%;
        margin-top:-50px;
}
100%{left:0px;
     top:0px;}



}