Is it possible to create a Script on Rule 34.XXX to search through your Favorites and does a CSS Image Border around all the images you have in your Favorites before you click on add this to your favorites button and seeing "Post already in your Favorites". R34 has a terrible search Feature but they have thousands of images that this site does not have. In fact i wish they would implant fav:user, and -fav:user already like you guys have. At 1st glance this site looks like it uses the Software/API does it?

From here :

e621's source code is a heavily modified fork of the Danbooru code. It is a Ruby on Rails application. It is not currently available to users.

Rule34.xxx is not affiliated with this site in any way, and if searching for stuff on their site is a nightmare because they have a terrible tagging policy, it's pretty much their problem. Even if that were the case, the public API for rule34.xxx is pretty much a joke, so the feature you're requesting would be impossible to implement as a user script very tedious to implement, since you'd basically need to have it load and parse the HTML for your favorites, which I'm sure some people would be motivated enough to try, but is definitely a lot less quick and easy to do than just using a public API and, since development is not open-source, I don't think that'll get added any time soon.

That aside, gelbooru (the framework rule34 appears to be running on) is said to be *inspired* by danbooru, but their public source releases show that it's really just a clone in PHP, so besides the overall look, it has pretty much nothing to do with danbooru or the in-house fork e6 uses.

if it makes you feel better you could download all the missing images and upload it to e621. the tagging is easy enough; just trawl through the wiki if you get any ideas.