[selectors] Pseudo-class for details

[AndySky21]

Pseudo-classes

Given the description for pseudo-classes and the fact that element interactivity is meant to be handled by UAs instead of relying upon scripts, it would be logical and consistent with other interactive elements if <details> open state were matched by an :open pseudo class.
On the other hand (and as stated by @chaals), this would would provide a CSS styling hook without the need for UAs to add/remove a content attribute according to user interaction. As a consequence, <details>.open IDL property will be allowed to reflect the element state instead of the content attribute.

Related issue: w3c/html#600

[annevk]

This shouldn't be needed as you can just use details[open] for this.

[chaals]

In w3c/html#600 I concluded that there is no real value in changing, so no real value in minting another pseudo-class...

[fantasai]

Alright, closing per @annevk and @chaals's comments. Going with invalid rather than wontfix because the premise of not being able to use [open] is invalid, but if you'd prefer it marked wontfix lemme know. ;)

@AndySky21 Let me know if this works for you!