I created a tutorial on the use of the <canvas> element in HTML.
The tutorial takes you step by step through the creation of a clone of the classic video game Breakout. At each step, you can edit the code of the game written so far, and immediately run it to see what the output is.
Click here to see the tutorial.
The code is taken from the presentation I gave in August to Refresh Baltimore, and the source code can all be found at my github repository.