[css-images] Lifting restrictions on nesting image-set()

Currently, the image-set() function forbids more image-set()s from
nested inside of it, either directly or inside of other <image> values
(like nesting within a cross-fade()).

On the call two weeks ago, I said that I don't think this restriction
is actually needed.  It was originally added due to the complexity of
handling fallback across nested things, but we removed the fallback
ability so that we could later produce a well-tuned fallback()
function or something similar that handled all the fallback properly.

Now, though, it shouldn't be hard.  Nesting them is still *weird*, and
there's no reason to do so, but there's no reason to *restrict* it,
which requires additional code to detect and enforce.

(I don't currently properly define how the resolution change actually
gets applied to the image; however I define it, some answer will fall
out for what nesting them means.  The actual answer isn't important,
because there's no use-case for nesting them.)

~TJ

Received on Thursday, 2 October 2014 20:57:47 UTC