Animation

Make the inside box move:

With a few changes (in green) to your stylesheet #outside {
height: 200px;
width: 200px;
background: yellow;
position: relative;
}

#inside {
position: absolute;
top: 0;
left: 0;
height: 10px;
width: 10px;
background: red;
animation: moveit 2s;
}

@keyframes moveit {
0% {top: 0; left: 0;}
100% {top: 50px; left: 50px;}
}