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"The Stage class represents the main drawing area and is a wrapper over the HTML5 Canvas element"

Judging by the example source they use canvas, but I dont know dart so cant tell if its one canvas or frame buffers in multiple canvases

For what I see on the Showcase games, they are canvas, but I really hope it supports webgl also, looks like an interesting library.

Perhaps showcase and documents section may answer some of your questions?