Why doesn't a ruleset for multiple ::selection selectors work?

Question

Why is it not possible to use the same ruleset for multiple ::selections as it is for other elements (eg div, span { }) ?

Why doesn't the second example work?

This works:

::selection {
 background: red; 
}
::-moz-selection {
 background: red; 
}

some text

Whereas this doesn't:

::selection,
::-moz-selection {
 background: red; 
}

some text

Why so?