While it's awesome that the new container shorthand grammar allows for ASCII-emojis in the middle of the declaration, it has some drawbacks as well:

• An empty value is a valid declaration: container:. (Equivalent to none / none).
• Values equivalent to none / none (the initial values) would serialize as an empty value (e.g. via gCS) per cssom shorthand serialization algos.
• Values equivalent to none / <something> would serialize as a string starting with /, which looks very odd. (E.g. container: / size).

It seems more normal to make the container-name part non-optional, i.e.:

  <container-name>? [ / <container-type> ]?
                  ^
                 Drop