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I knew about the A vs. Α vs. А issue, where visually similar/identical characters map to different domain names. But I didn't know IDNs also could map visually different characters to the same domain names. I would've guessed that full-width characters would be punycoded as well, rather than treated as their ASCII equivalents. Is this done with any other characters?

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Perhaps this is not so surprising. Prior to IDNs, the DNS also did case folding so "a" and "A" would go to the same place.

One of the particular challenges with IDNs is that there are two versions of the specification, a deprecated 2003 version and the current 2008 version. For a few characters they provide subtly different transforms. The 2008 version also ratchets down on a lot of non-sensical characters — they are no longer eligible in domain names. The remaining permissible set is quite conservative to limit some of the issues seen in the original version.

This has nothing to do with UTF-8. Most of his examples use Punycode, the filename one is UTF-16.

Don't confuse Unicode with how to represent code points in bytes, please.

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