When designing grammars with <custom-ident>, the <custom-ident> should always […]

This is a "should" requirement for CSS specifications, which Grid happens to violate since span in some positions is ambiguous.

The part of https://drafts.csswg.org/css-values-3/#custom-idents relevant to implementations (though it may have changed since previous comments here) is:

When parsing positionally-ambiguous keywords in a property value, a <custom-ident> production can only claim the keyword if no other unfulfilled production can claim it.

For example, the shorthand declaration animation: ease-in ease-out is equivalent to the longhand declarations animation-timing-function: ease-in; animation-name: ease-out;. ease-in is claimed by the <easing-function> production belonging to animation-timing-function, leaving ease-out to be claimed by the <custom-ident> production belonging to animation-name.

This says that span span should not be a parse error. Instead the ambiguity should be resolved by making the first span resolve as a keyword (since that grammar production is at that point unfullfilled), and the second one as a <custom-ident> (since by then that is the only remaining grammar production).

This "priority" is more important in e.g. span 2 where we want to parse the span keyword and not a custom identifier.