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

Code css:
body {
  font [
    face: Arial;
    size: 2em;
    style: italic;
    weight: bold;
  ]
}

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...

Code css:
body {
  background: [
    image: url(some/path);
    opacity: .2;
    color: #fff;
    position: fixed;
    size: 50;
  ], [
    image: url(some/path);
    opacity: .2;
    color: #fff;
    position: fixed;
    size: 50;
  ]
}

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.

Code css:
div {
  border [
    width: 1em;
    style: solid;
    top [
      width: 2em;
    ]
  ]

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).