Problem A
The issue is that for some syntax reasons, stylus doesn't permit mixins alone in a selector
The solution(s)
is to use multi line
.red { opacity(0.4); }
add a bogus property (make sure it doesn't affect your styling)
.red { opacity(0.4); zoom:1; }
Problem B
Another issue was with reset styles, being without a new line between them.
body{line-height:1}ol,ul{list-style:none}blockquote,q{quotes:none} ...
The solution
To put each of the style on individual lines:
body{line-height:1}
ol,ul{list-style:none}
blockquote,q{quotes:none}
...
Very very weird issues and even weirder solution :P
Hope that this save some of your time (because I've wasted a lot of mine on this :( ).