XML and XMLName should account for U+061C (ALM)

[aphillips]

Proposed comment

https://www.w3.org/TR/REC-xml-names/#ns-qualnames
https://www.w3.org/TR/xml/#sec-common-syn

[4] | NameStartChar | ::= | ":"  \| [A-Z] \| "_" \| [a-z] \| [#xC0-#xD6] \| [#xD8-#xF6] \| [#xF8-#x2FF] \|  [#x370-#x37D] \| [#x37F-#x1FFF] \| [#x200C-#x200D] \| [#x2070-#x218F] \|  [#x2C00-#x2FEF] \| [#x3001-#xD7FF] \| [#xF900-#xFDCF] \| [#xFDF0-#xFFFD] \|  [#x10000-#xEFFFF]
[4a] | NameChar | ::= | NameStartChar \| "-" \| "." \| [0-9] \| #xB7 \| [#x0300-#x036F] \| [#x203F-#x2040]
[5] | Name | ::= | NameStartChar (NameChar)*
[6] | Names | ::= | Name (#x20 Name)*
[7] | Nmtoken | ::= | (NameChar)+
[8] | Nmtokens | ::= | Nmtoken (#x20 Nmtoken)*

XML 1.0 5e and XML Names 1.0 use the construct NameStartChar shown above. The characters in names defined using NameStartChar were deliberately limited to avoid known problem characters at the time of adoption. Notable among the characters excluded from names are invisible formatting controls.

The character U+061C ARABIC LETTER MARK was added to Unicode in version 6.3 (in 2013). This character is similar to U+200F RIGHT-TO-LEFT MARK, which is not a NameStartChar. It is unusual that an invisible, non-spacing mark like this be added to Unicode. An XML name that consists of this single, invisible formatting control is thus valid, but it seems like a bug, not a feature.

(This issue was encountered in creating the MessageFormat 2.0 standard at Unicode, where we are attempting to use NCName and Name to define valid identifiers).

The downside, of course, is that very many implementations will not be aware if a change were made to NameStartChar.

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