CSS

Transitions

#box.left {
  margin-left: 0;
}
#box.right {
  margin-left: 1000px;
}

document.getElementById('box').className = 'left'; 
document.getElementById('box').className = 'right'; 

#box {
  -webkit-transition: margin-left 1s ease-in-out;
}

document.getElementById('box').className = 'left'; 
document.getElementById('box').className = 'right';