Flex 4 descendant pseudo selectors with CSS

Question

Flex 4 offers descendant css selectors and pseudo-selectors like:

s|DropDownList:open {
    font-size: 11;
}

s|DropDownList #labelDisplay {
    font-size: 12;
}

However, if I combine these two an wanted to do something like this, it doesn't seem to work:

s|DropDownList:open #labelDisplay {
    font-size: 13;
}

Is this possible with descendant pseudo-selectors?

(Of course I could use a styleName.open property on labelDisplay, but I think using pseudo-selectors is a more elegant solution)