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Expected output:

.foo .bar{
  color: #333;
}
.foo.zoo {
  color: #222;
}

In Sass/Less/Stylus:

.foo {
  .bar {
    color: #333;
  }
  &.zoo {
    color: #222;
  }
}

In CSS nesting (the new CSS spec feature):

.foo {
  & .bar {
    color: #333;
  }
  &.zoo {
    color: #222;
  }
}

For the new CSS spec, IMO, & is just simply appending everything following to the selector in upper level.