style.css

*{
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body{
  background: #F7F7F7 ;
  overflow: hidden;
}
.hdng{
  margin-left: 120px;
  margin-top: 50px;
  margin-bottom: -70px;
  font-size: 40px;

}
#screen{
  margin: 150px 10%;
  width: 80%;
  height: 400px;
  border: 5px solid green;
  position: relative;
}
#character{
  position: absolute;
  top: 260px;
  border: 1px solid red;
}
.animate{
  animation: jump 1s;
}
#character img{
  width: 121px;
}
@keyframes jump {
  0%{
    top: 262px;
  }
  30%{
    top: 100px;
  }
  70%{
    top: 100px;
  }
  100%{
    top: 262px;
  }
}
#cactus{
  position: absolute;
  bottom: 0;
  right: 0;
  animation: animate 2s linear infinite;
  border: 1px solid blue;
}
#cactus img{
  width: 64px;
}
@keyframes animate {
  0%{
    left: 750px;
  }
  100%{
    left: 0px;
  }
}
hr{
  position: absolute;
  bottom: 48px;
  width: 100%;
  background: #505050;
  height: 3px;
}
.over{
  display: none;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
  font-size: 30px;
  color: red;
}