I don't know much of anything about Canvas, but it looks way too involved for different implementations to ever have any hope of reliable compatibility.
Like anything ever, there'll be a reliably-implemented common core, and a variety of corner-cases that are handled differently across implementations.
For example, Firefox fails some of those tests because if you call `canvas.getContext()` and pass a gibberish context-name, it throws an exception rather than returning `null`. Once there are any standardised context-names besides "`2d`", this might be a problem; I expect Mozilla will have fixed it by then.
No, the fact that it has been specced out with repeatable, automated tests means that you have much more hope of reliable compatibility than say CSS 2 where it was just text and everyone interpreted it differently and there was no "correct" answer to compare to.
Or just like most of Javascript, the big library makers (Prototype, JQuery, YUI) will implement a nice wrapper that hides all the implementation details for you and lets you get on with your life.
I don't know much of anything about Canvas, but it looks way too involved for different implementations to ever have any hope of reliable compatibility.