[css-compositing] should background-blend-mode be reset by the background shorthand?

Should the background-blend-mode property be reset by the background
shorthand?  I'm inclined to think that it should be.  All of the
other per-background-layer properties are, and in the future we
might want to allow setting it in the shorthand (which would require
that the shorthand resets it).

-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)