On the Android app for Wikipedia, when viewing an article, there's an option to "Add to Reading List". Is this feature available in the web version of Wikipedia? I haven't been able to find it.
This feature is currently available only for Android.
Quoting from Wikimedia_Apps/Android_FAQ:
(The app) "... It adds features that are not available (or not possible) in the Web version of Wikipedia, such as saving pages to the device for later reading..."
"A synced reading lists feature is currently in Beta"
When will synced reading lists be available on other platforms?
Reading list sync is currently only available on Android, though we anticipate that the feature will soon be available in the iOS app as well. While there are no immediate plans to add the sync feature to desktop or mobile web, we encourage you to occasionally check back for further updates.
As a conclusion - it's not available for the desktop version, nor there are immediate plans to add it to desktop.