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In Serialization 3.1: "Escape according to the rules of the XML, HTML, or JSON output method, whichever is applicable, any characters (such as < and &) where XML or HTML requires escaping, any characters (such as " and `) where JSON requires escaping, and any characters that cannot be represented in the selected encoding." In RFC7159: "All Unicode characters may be placed within the quotation marks, except for the characters that must be escaped: quotation mark, reverse solidus, and the control characters (U+0000 through U+001F). " I think ` is a typo and \ was intended.
Fixed in latest draft... https://www.w3.org/XML/Group/qtspecs/specifications/xslt-xquery-serialization-31/html/Overview.html#serphases