Intent is to make all headers list-based.

[mnot]

A start on #193.

[royfielding]

Umm, no way. Uh uh. -1.

The ABNF is for generating fields. Singletons are an instruction not to generate more than one. It is impossible, right now, to make that change and expect conforming implementations to magically adapt. Multiple instances of singleton field values are non-interoperable and will remain so for the life of HTTP.

A parser implementation might want a set of robustness rules for multiple field handling, regardless of their generative definition. But the security of those rules are going to depend on the semantics of the field (e.g., Host, Auth, C-L).