La especificación Atom10 requiere que 'X' tenga uno de estos valores: "text", "html", "xhtml"; no obstante, este valor es 'Y' en el documento que se va deserializar.
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English version
The Atom10 specification requires 'X' to have one of these values: "text", "html", "xhtml", however this value is 'Y' in the document being deserialized.