These were always invalid per spec, but every browser parsed them.
Chromium was fixing a few of their bugs in the area (see
https://github.com/w3c/csswg-drafts/issues/4042), and they made the
change to reject negative lengths as well in:
I think it makes sense, and somebody else independently filed:
So I think it's worth trying to get the change here.
Note that this affects both prefixed and modern gradient syntax.
Relatedly, recently people found interesting compat bugs in legacy
gradients ... I may land a fix for that in the near future but that's
not the point of this intent (that has ~no potential of breaking sites
so I don't think it's worth an intent, so it's basically a bugfix).
There are WPTs for this change, and I didn't see a WebKit bug so I filed
one at https://bugs.webkit.org/show_bug.cgi?id=202412.
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