#5831 introduced url-set definitions of the cursor property. However, it also introduced a subtle change in the 78fb03b#diff-cc2f666bf7d93dcefcc3fffc4761793a54fba00c03b53b9dbdf81f5056f0b56eR604

before:

[ [<<url>> [ <x> <y> ]?,]* [ auto | … ] ] 

after:

[ [ <<url>> | <<url-set>> ] [ <x> <y> ]? ]#? [ auto | … ]

While looking very similar, the two versions are not equivalent: the former requires a comma after each URL, resulting in a comma between the last URL and the "fallback" predefined cursor (e.g. as in cursor: url(example.png), auto). The latter does however use the #¹ syntax which does not allow for a comma to be present after the list of URLs (e.g. hypothetically resulting in cursor: url(example.png) auto).

As latter (#?) version is not backwards compatible to CSS Level 3, incompatible with how browsers currently handle the definitions of cursors using URLs and does not match the example given on the current draft version of the page as well as other related sources like wpt, I assume this change was introduced by mistake.

PS: with 1a57a40, this mistake also made it into the CSS Level3 definitions on this repository.