The explanation in your blog post isn't quite correct. The conversion of "Ⅷ" to "VIII" happens when Unicode text is normalized using a "compatibility" mapping,

The explanation in your blog post isn't quite correct. The conversion of "Ⅷ" to "VIII" happens when Unicode text is normalized using a "compatibility" mapping, that is normalization forms NFKC or NFKD, not when text is lowercased. While IDNA2003 required a custom mapping based on NFKC, IDNA2008 doesn't specify a mapping phase. It does allow custom mappings, though, and it seems that browsers apply NFKC at some point in the process. See https://unicode.org/reports/tr46/#Mapping https://unicode.org/reports/tr46/#Mapping