Proposal: make a consistent "list" syntax in CSS, and list of lists syntax in CSS.

In this post that I wrote up a little while back, I talked about "The Many Failed Attempts at Lists-of-Lists Syntax in CSS". Right now, CSS is scattered with various attempts to syntactically define "lists-of-lists" syntax, and the recent CSS Functions and Mixins module is currently on-track to introduce another lists-in-lists syntax attempt, which, via the use of curly-braces, rolls back previous understanding of list syntax, where curly braces, by convention, only held declaration lists.

I don't have a specific proposal for what lists-of-lists syntax should look like; I just think it's worth CSSWG exploring a single CSS syntax to represent it, and to:

1. use a consistent list syntax and lists-of-lists syntax moving forward, and
2. backporting, as needed, list syntax and lists-of-lists syntax to previous lists-of-lists syntax attempts, for consistency

If I had a preference, it would be:

1. curly braces only hold declaration lists, or
2. curly braces are used everywhere there are lists-of-lists, or
3. a previous lists-of-lists syntax is used for new proposals, or
4. a new lists-of-lists syntax is applied that is used for new proposals and backported to previous syntax

In other words, my preference for CSS would just be consistency.

Start with the linked post and the current CSS Functions and Mixins module discussion. Compare the existing list and lists-of-lists syntax conventions described there, then summarize possible consistency directions for CSSWG review. Done means the working group has selected or clearly documented a consistent syntax direction and its compatibility implications.