Shortest Pong, AI vs AI
Pure HTML and CSS, 355 characters

&lt;div class=p&gt;&lt;/div&gt;&lt;div style="height:300px"&gt;&lt;/div&gt;&lt;div class=p&gt;&lt;/div&gt;&lt;div id=b&gt;&lt;/div&gt;0-0&lt;style&gt;.p{width:99px;height:20px;background:#000}*{margin:0}#b{position:absolute;background:#F00;width:20px;height:20px;animation-name:b;animation-duration:3s;animation-iteration-count:infinite;left:40px}@keyframes b{from{top:20px}50%{top:300px}to{top:20px}}&lt;/style&gt;


You may need an up-to-date or beta browser for the CSS animation properties.

Here is a fiddle, with -webkit- vendor prefixes added so that it works in latest Chrome.



The ball just bounces back and forth. The AIs are extremely smart and know not to move.

The 0-0 at the bottom is the score, and you can resize the window.