[whatwg/url] IDNA / UTS #46 "should" requirements (#110)

https://url.spec.whatwg.org/#idna refers (through the “Unicode ToAscii” and “Unicode ToUnicode” algorithms) to http://www.unicode.org/reports/tr46/#Processing and rely on the error flag.

>  The following steps, performed in order, successively alter the input domain_name string and then output it as a converted Unicode string, plus a flag to indicate whether there was an error.

This it turns refers to Section 4.1 http://www.unicode.org/reports/tr46/#Validity_Criteria which has a series of “must” requirements. For example:

> The label must be in Unicode Normalization Form NFC.

This section also has a subsection 4.1.2 http://www.unicode.org/reports/tr46/#Right_to_Left_Scripts

>  In addition, the label **should** meet the requirements for right-to-left characters specified in the Right-to-Left Scripts document of [IDNA2008], and for the CONTEXTJ requirements in the Protocol document of [IDNA2008]. It is strongly recommended that Unicode Technical Report #36, Unicode Security Considerations [UTR36] and Unicode Technical Standard #39, Unicode Security Mechanisms [UTS39] be consulted for information on dealing with confusables, and for characters that should be excluded from identifiers. Note that the recommended exclusions are a superset of those in [IDNA2008]. 

Note “should” (emphasis added) and “strongly recommended” rather than “must”.

If the URL Standard is to define interoperable algorithms, I think it needs to define in which requirements Section 4.1.2 sets the error flag.

(Related: https://github.com/servo/rust-url/issues/179)

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