Zero-width characters are invisible, ‘non-printing’ characters that are not displayed by the majority of applications. For example, I’ve inserted 10 zero-width spaces into this sentence, can you tell? (Hint: paste the sentence into Diff Checker to see the locations of the characters!). These characters can be used to ‘fingerprint’ text for certain users.
A cool little trick with text for embedded steganography, security, or other communication purposes.
This could also be used for pseudo-private communication via Webmention even. Just hide your messages inside of public messages.