Earlier quoted context omitted.
>Why, to center an object, do I have to remember trivia like "the parent needs a width." Because you're still viewing the page as a table. Once you stop doing this, you stop asking questions like these.
I'm open to new perspectives. In what way I should I see the page such that "center me in my parent" is unreasonable unless the parent has a fixed width? What, in particular, about the box model makes this impossible?
Now, you could give the object in question a width and simply give it margin-left/right: auto. That would center it in the width-less parent, but it wouldn't fix its position to the particular spot you're currently seeing it in.
For understanding, take it up to the highest level, the browser window. Make your browser 1024x768. Now make it bigger or larger - where is the "center"? It's moved depending on the size of the browser window.
If you have a parent whose width is variable, you can't center the child. The key is that, unlike years of using tables to hack up a website, the canvas CSS uses is variable; its height and width is undefined. Thinking of it like this, it makes perfect sense that center isn't used.