It might not be compact but the gzipped size is the same.

    $ cat <<EOF | gzip | wc -c
    ul li:nth-child(1) {
      background: red
    }

    ul li:nth-child(2) {
      background: red
    }

    ul li:nth-child(3) {
      background: red
    }
    EOF

    71 # bytes

    $ cat <<EOF | gzip | wc -c
    ul li:nth-child(1),
    ul li:nth-child(2),
    ul li:nth-child(3) {
      background: red
    }
    EOF

    71 # bytes