A minor issue, but it differs from ruby-sass. Consider this test case:

#foo {
  d: .0001px;
  f: .01px;
  d: -.0001px;
  f: -.01px;
}

The expected result after passing through sassc is the the identity. I actually got this instead:

#foo {
  d: 0.0001px;
  f: 0.01px;
  d: -0.0001px;
  f: -0.01px;
}

According to CSS 3, 4.2. Numbers: the number type, a number is either an integer or zero or more integers followed by a dot, followed by one or more digits.