I don't think square braces are used anywhere in the css spec. A thought occurred to me this morning - why not use them for subproperties - like this
Yeah, we have property shorthands, but for some modules this is getting out of hand. In particular backgrounds. When a property can have multiples they can be comma separated...
The idea further is that these could be nestable as seen here with border, where the top property has a more specific call than the other three.
Just a thought. Now someone can come in and tell me why this is utterly stupid and unworkable
BTW, this isn't a replacement for multi-property shorthand (for example: border: 1px solid #ff0). It's a more approachable, legible alternative (as it doesn't require the reader to have memorized the argument order).