W3Cの「Technical Report」の中に、Shift-JISをCESとして扱う、と書いてある。
XML Japanese Profile
This technical report and [XML] treat Shift-JIS, an ordinary Japanese charset in Japan, as a CES that represents Japanese characters and [US-ASCII] characters in [ISO/IEC10646] or [Unicode 3.0]. For full interoperability in the Internet, migration from Shift-JIS to UTF-8/UTF-16 is highly recommended.
日本語版(オリジナル？)
標準情報(TR) TR X 0015:2002 XML日本語プロファイル
http://www.y-adagio.com/public/standards/tr_xml_jpf2/toc.html