As I understand this project is about registering standard applications for browser and email so that other applications do not have to bother with this. I would suggest to extend this standard in a way that it also determines the standard feed reader which is used for feed subscription. I am using quite often Firefox on a KDE-system. If there something like a "xdg-feed-subscribe <feed>" Firefox would not need to bother anymore and users would not need to bother about how to make Firefox subscribe feeds to akregator.
Given the rate of dropoff in RSS over the past few years, I think this is now very unlikely to happen.
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