What if you want an HTML-first page with a canvas in it, but then you realize you want some layout/styling for the text within the canvas? Seems unnecessary to propagate that situation up to the type of top-level page.
And then what if you realize you need a canvas-in-the-html-in-the-canvas? It's endless. Canvas-first makes sense, it's basically how it works everywhere outside of the web. Start with the smallest abstractions and build on them (html on canvas) rather than leave escape hatches to your big abstractions because they fail to cover every use cases (canvas in html).
If you support the DOM and hitscan, then it doesn't matter. You can red pill Ouroboros yourself all day and not care. Every element a canvas, every raindrop an ocean.
