This is something that keeps me worried at night. Unlike other historical artefacts like pottery, vellum writing, or stone tablets, information on the Internet can just blink into nonexistence when the server hosting it goes offline. This makes it difficult for future anthropologists who want to study our history and document the different Internet epochs. For my part, I always try to send any news article I see to an archival site (like archive.ph) to help collectively preserve our present so it can still be seen by others in the future.
I guess I can talk a bit about the first and third points for my personal archiving (certainly not on a global scale).
For data storage data should be regularly be checked for bitrot and corruption, preferably with a file system that can heal itself if such a situation occurs. Personally I use ZFS RAIDZ with regular scrubs to sure that my data is bitperfect. Disks that regularly show issues are trashed, even if they appear to run fine and show good SMART status. For optical disks in a safe or something I reburn them every ten years or so even if they’re still readable to keep the medium fresh.
I’ve actually known someone who had to painfully setup a Windows 95 computer in order to convert some old digital pictures from a equally old digital camera stored in a prop format. Obviously that’s a no go. For my archives I try to use standard open formats like PNG, PDF, etc. that won’t go away for a long time and can be reconverted as part of an archive update if the format starts to become obsolete. You can’t just digitally archive everything and expect it to be easily readable after a hundred years. I don’t do this but if space is limitless lossless format could be used (PNG for photos, FLAC for audio, etc.) so any conversions remain true to the original capture.
Actually I think TIFF or Adobe DNG are the lossless formats for photos.
TIFF is a classic storage format, but PNG is common for web images and isn’t going away either. DNG is for RAW sensor output from professional cameras and is not used for edited and published images. However if you’re archiving your photo collection or something than keep the DNGs!