[css-content][css-pseudo] alt missing from generated content module? moved?

Example 21 and 22 in the "Automatic Counters and numbering section"

https://drafts.csswg.org/css-content-3/#counters

...reference the "alt" property that was previously defined in this spec. 

Will someone point me to the new location of that property definition? 

I could not find it in Pseudos either.
https://drafts.csswg.org/css-pseudo-4/