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http://url.spec.whatwg.org/#idna [[ Let result be the result of running Unicode ToASCII with domain_name set to domain, UseSTD3ASCIIRules set to false, processing_option set to Transitional_Processing, and VerifyDnsLength set to false. ]] The "Unicode ToASCII" link is a dead link so not possible to see what that step means.
Sorry about that. Seems Unicode has been updated. https://github.com/whatwg/url/commit/5142bf8649f505f564ea599dd38e0bc8cb1d166c