Currently, Page Previews will load and run on all pages though, fortunately for Reading Web, the links that it works on don't tend to feature on pages like Special:UserLogin or Special:Preferences. Page Previews shouldn't be loaded on a limited subset of pages to avoid any ambiguity.
To be clear, this task isn't about limiting requesting previews for pages, e.g. not requesting previews for special pages; it's about limiting the pages that the Page Previews code is loaded on.
== AC
[x] A configuration variable holds a blacklist of pages that Page Previews must not be loaded on, e.g. `wgPopupsPageBlacklist`.
[x] It defaults to Special:UserLogin and Special:CreateAccount
[x] If a user (logged in or out) navigates to a page in the blacklist, then they aren't delivered the PP code.
== Closed Questions
[x] Should this be a blacklist?
@phuedx: Yes. You're absolutely right.
[x] Can we use wgContentNamespaces?
@phuedx: I'd like Page Previews to work in `NS_SCHEMA` for example. As @Jdlrobson says, the number of places where we want this to run far outweighs the number of places where we don't.
[x] What should the default value of the blacklist be?
@phuedx: Initially, `[ 'Special:UserLogin', 'Special:UserLogout' ]`.
== Developer Notes
1. For performance reasons, we should not be delivering the PP code when we don't have to.
== Before sign off
[] Update https://www.mediawiki.org/wiki/Beta_Features/Hovercards with the final list of blacklisted pages.
[] Add a note to [[ https://meta.wikimedia.org/wiki/Schema_talk:Popups | Schema_talk:Popups ]] explaining that data won't be collected from blacklisted pages.