They all either are or can be strict supersets of CSS3. What separates stylus is that you can write valid, non-superset CSS3 and invoke the appropriate functions/mixins:

    @import css3

    #transparent {
        opacity: 0.6;
    }

    ----

    #transparent {
      opacity: 0.6;
      -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=60)";
      filter: progid:DXImageTransform.Microsoft.Alpha(Opacity=60);
    }

That's input/output in stylus. This uses an `opacity` mixin defined that's being invoked without the optional parentheses and with the optional colon+semicolon.

I have the beginnings of the Compass CSS3 library that's on hold while TJ fixes bugs and adds features to cover edge cases. The eventual goal is the ability to write straight CSS3 and have everything work.