I have used the canvas element and it runs fine in all modern browsers.
Click on “open midi configuration” first to connect a keyboard and a midi output, then click “close midi configuration” and play your keys: enjoy the beautiful rainbow colors while playing!
For more information and links about the colors and music see this blog.
Note for Internet Explorer 7 and 8 users: to support the Canvas element in older browsers i use Googles Explorer Canvas, and thus VML. Because VML is quite slow, you can run the app in Internet Explorer 7 and 8, but it is actually too sluggish for real time animations like this.