It would be nice for the user to have an instant feedback on their action.

Here are a few examples:

1. User opens an item in their feed, then press the envelop (or associated keyboard shortcut) to mark this item as unread
2. User clicks on the star (or use associated keyboard shortcut) to mark an item as favorite

Expected results:

1. The item is immediately marked as unread (the envelop icon and the item color change to reflect this)
2. The item is immediately marked as favorite (the star icon changes to reflect this)

Acual results:

1. There is a delay before the item is marked as unread
2. There is a delay before the item is marked as favorite

Now, I understand the delay is caused because when clicking on the envelop, the page sends a request to the server, waits for the server to acknowledge and make sure everything is fine, and then finally provides visual feedback.

In the case where something goes wrong (for instance the server cannot be contacted), a flow such as the following could be used:

1. User clicks on the envelop to mark item as unread
2. The visual feedback is immediately made (envelop icon changes, background color changes)
3. The page realizes the server cannot be contacted and times out (or any other communication error)
4. The item state is reverted in the page (since it could not be updated on the server) and an error popup is displayed.

This is actually something that is implemented in Selfoss (another open source feed reader), so for implementation details, tips might be found in its source code :)