This is not about Canvas but rather set of primitives that browser uses for rendering web pages.

Like in my Sciter (https://sciter.com) where you can do

    var anyEl = …;
    
    anyEl.paintBackground = function(gfx) {
      gfx.fillColor(rgb(0,0,0))
         .rectangle(0,0,100cm,100cm);
      return true; // to suppress CSS background drawing 
    }

paintBackground/Content/Foreground methods are called while rendering HTML tree and the `gfx` passed to that function is the very same graphics that is used for rendering HTML/CSS tree.

That thing allows to combine immediate mode graphics a la ImGui with retained one to achieve more performant solution at the end.

The same approach can be used with WASM I think.