It was resolved here to have both :open and :closed pseudo-classes: #7319 (comment)

@annevk mentioned here that :closed is redundant with :not(:open): WebKit/standards-positions#413 (comment)

Here is the justification for having both of them: #7319 (comment)

Should we also talk about adding the corresponding :closed pseudo class? That would avoid the problem that :not(:open) can match anything, including things that don't open or close.

The HTML spec has not been merged yet: whatwg/html#10126