Is your feature request related to a problem? Please describe.
While certainly helpful, the current implementation of favorites lacks some depth to be seriously used for tracking your BiS items:

• There is no option to write Notes for items
  • This makes adding multiple items for the same slot very confusing (like BiS + Alternatives)
  • Allows to quickly judge how helpful the upgrade would be right now (e.g. Set-Bonus counts or Hit-/Expertise-Cap dependencies)
• In the "item browser" you can't tell where you have favorite items defined
  • Limits usability for determining what instances/raids you need items from
  • Makes it hard to create prio-lists from favourites
• There is no difference between received/obsolete items and pending upgrades
  • Requires you to delete received/lower-tier items every time you receive an item to keep the favorite list useful
• Import/Export favorites including the defined notes
  • Easily share/import BiS-Lists ready to use
• The current Import/Export function requires transmitting the data using a third-party application, chat-links would make sharing lists much easier

Describe the solution you'd like
I've implemented (imho) a good baseline to bump the favorite feature to a really helpful state. This includes:

• Notes to favorite items
  • You can add/change favorite notes by alt-right-clicking a favorite item
  • If defined the note is shown in the item tooltip along with the according favorite list name
• Favorite counts to the various selections of the "item browser" to quickly identify where you still need items from
  • Instances/Bosses/Difficulties/... now include a favorite count (including a per-list breakdown in the tooltip)
  • Items that you already received will not be counted
  • Items you already own a higher item-level alternative for are not counted (except if defined as "main item" in the favorite list)
• Items that are already owned or obsolete are filtered
  • This scans your current equipment, wow equipment manager and outfitter profiles to figure out which items are no longer needed
  • Items you already own will be greyed-out in the tooltip with the suffix "(owned)" and no longer be counted in the "item browser"
  • If you already own a higher item-level item for the same slot (and the item is not defined as a "main item" in the favorite list) it will be greyed-out in the tooltip with the suffix "(obsolete)"
• Import/Export favorites including notes
• Simple sharing function using chat-links (Uses the existing Import/Export functions for transmission)

Pending improvements/features

• Outfitter integration is no yet optimal
  • In my case Outfitter is loaded after Atlasloot which requires a small workaround to obtain the required data. It works, but could probably be solved better (see PopulateListBiS() in Favourites.lua)
• The favorite counts in the "item browser" do not refresh instantly when adding/removing favorite items
  • I tried to trigger a full refresh (in Favourites:OnItemsChanged()), but all methods I tried resulted in switching to a different boss. So I reverted it for now and noted it here.
• Detecting owned/obsolete items certainly ain't perfect yet (in Favourites:IsItemEquippedOrObsolete() / PopulateListBiS())
  • Support for ItemRack would be helpful for those relying on that
  • Slots with multiple items (rings/trinkets/1h-weapons) could use a special rule to improve results
  • Manually adding "BiS"-Items with e.g. a keyword in the note could help prioritization
• Import/Export favorites including the defined notes
  • Check the current format for notes and change if it clashes with the planned formatting
  • Include "main items" in the export
• Order the items by "required" > "owned" > "obsolete" in the favorites window

Screenshots

• Setting/Updating favorite notes

• Favorite counts in the "item browser"

• Variants of item tooltips (needed/owned/obsolete/multi-list)

Added sharing lists via chat-links.

I've also written an online converter for a few pages for BiS-Guides/Lists here: https://atlaslootclassic.wow-bis.de/

Not sure how good the acceptance of such a tool would be, but I'll certainly use it for wowhead-guides from our feral lord and savior NerdEgghead :D

• www.wowhead.com should work completely for all item options with according notes and even has a configuration section to define how the various tables should be imported.
• wowtbc.gg Edit: Works all the way now.
• www.icy-veins.com only the BiS-Items, since there is nothing else.
• Other pages should work as well as long as the items are wowhead-links

I'd have to figure out which license makes sense for publishing the source, since it uses and contains various libraries: Bootstrap, jQuery, Twig and a blizzard api-library. (I think I did write that one a long time ago xD)

It works by parsing the html from the URL and converting that into an Atlasloot Import-String. For Eightyupgrades.com it would probably make sense just to display a textarea for it's export string and do the conversion in javascript.

If there's need for it I can make sure at least the most common pages for BiS-Lists work, otherwise I'll keep it as is and only improve/fix what annoys me personally.