They are no longer supported by sass and broken with libsass 3.6.3 (https://github.com/sass/libsass/issues/3033)
This removes some of them by replacing them with a placeholder selector. This at least brings the resulting CSS size down a bit so gtk can be build again.
The remaining cases I don't know how to convert because I haven't found a way to reproduce the old output.
The CSS was generated with libsass 3.5.5.
See #2237 (closed)