Result depends on CSS Sequence when :fullscreen is used.

[oleersoy]

This works:

.a
  color: red
:fullscreen

It builds the way this article describes it: https://medium.com/@ole.ersoy/building-sugarss-with-superflycss-b38a09d17930

Or like this:

.a { color: red }
:-webkit-full-screen {}
:-moz-full-screen {}
:-ms-fullscreen {}
:fullscreen {}

However if the selectors are flipped:

:fullscreen
.a
  color: red

The result is this:

:-webkit-full-screen
.a {
  color: red
}
:-moz-full-screen
.a {
  color: red
}
:-ms-fullscreen
.a {
  color: red
}
:fullscreen
.a {
  color: red
}

[ai]

Yeap, it did it by design. Think feature allow you to have multiline selectors.