grumbles about content-empty words like “mode” Oh, all right. :p

I think this would be slightly different than background-blend-mode, because with background-blend-mode the blending only happens among the background layers of a single element, and there is an implicit isolated group around all background layers of the element. So no blending happens with anything behind the element.
But for fill-blend-mode and stroke-blend-mode you probably want to blend with content behind the element.

@mstange

I don't see a problem with isolating the element, but I would want the shape as a whole (fill, stroke, and markers, after paint-order re-ordering) to be the isolated group. So a stroke layer could be blended with the fill, or markers with the stroke. (If you didn't want them to blend, you would just give the bottom layer normal mode.)

Once the element as a composite, isolated group layer has been painted, then mix-blend-mode would control how it blends with the background.

(The alternative would be to have separate properties for blending within the fill/stroke stacks versus blending between them, and I think that overly complicates things.)

Adding normal to the bottom layer would not isolate the entire group. Top layers would blend with the bottom layer and the canvas below it. Imagine a half transparent bottom fill or a partly transparent pattern and you'll notice the difference.

That said, the isolation property would isolate the entire group. IMO @mstange does have a good point that we might rather want to have non-isolated blending by default.

@fantasai Would new blending properties for fill and stroke go into the Compositing spec?