Optional child selector, with CSS/Stylus/LESS: .aa > .bb? > .cc

if you want use your option, by the way is very interesting, you can use a form most correct:

>.aa (>.bb)? >.cc { ... }

but the form correct would:

.aa .cc { ... }

into stylus:

.aa   
  .cc

thats all.

Wouldn't .aa > .cc, .aa > .bb > .cc {} work? Or did I misunderstand your question?
This selects only the .cc that are direct .aa children and the .cc that are .bb children (children of .aa) as well.

For Stylus and Sass you could do this (live example for Sass):

.aa
  > .bb, &
    > .cc
      width: 10px

I couldn't find a way to do so in a one-liner for Sass, but for Less/Stylus/Scss you could do also this (live examples for Scss, for Less) :

.aa { > .bb, & { > .cc {
  width: 10px
}}}

This is not that pretty also, but still better than nothing :)