Flash posts that have been manually confirmed to work with the version of the Ruffle.js emulator that's currently used by Danbooru (you can right-click on any Ruffle player window to see the version number). If you happen to be running the browser extension version of Ruffle.js, which sometimes overrides the versions that are hosted by the websites themselves, please turn the extension off and verify that a post still works with Danbooru's current hosted version before adding this tag to the post.
Posts with visual errors still qualify for this tag, as long as it plays and is still generally viewable (and playable, in the case of Flash Games).