A while ago, I thought it would be fun to play around with HTML5 Canvas a little bit. I came up with Canvas Pong.

The way it works is by giving a 100% x 100% <canvas> element absolute positioning on a page with a flexible height. Using the height in pixels (which can change depending on difficulty) and the current scroll position, I can easily work out the size of the right paddle. Once I had that going, I just constructed (with the 2D Javascript drawing API and pure DOM manipulations) a bar on the other side that matches the height of your paddle, and a ball to move around.

Definitely not meant to be performant, but it was a really good project to play with <canvas>. You can take a read of the code by viewing source, or on the GitHub project page. Think up a project of your own and see what you can make.