[css-masking] when to interpret 'mask-image' as SVG mask vs. image?

https://drafts.fxtf.org/css-masking-1/#the-mask-image defines a
property that can use the url() function to link to either an SVG
mask element or to an image.

I don't see where it specifies how to tell which one of these two to
process a link as.

This ought to be defined clearly.

(I have vague memories of it being discussed in a working group
meeting, but I couldn't find the minutes.)

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)