Because I only showed it the CSS! It doesn't even get the HTML, it's guessed all of that exclusively from what's in the (uncommented) CSS code.

Though it's worth noting that CSS Minecraft was first released three years ago, so there's a chance it has hints about it in the training data already. This is not a meticulous experiment.

(I've had a search around though and the most detailed explanation I could find of how that code works is the one I posted on my blog yesterday - my hunch is that it figured it out from the code alone.)