1. A computer system comprising: a sending computer including an encoder module encoding an internal representation of a structured document to generate an encoded document; and a receiving computer including a decoder module which receives said encoded document through a network, and decodes said encoded document to reproduce said internal representation, wherein said encoder module lists texts and structures included within said structured document without duplication to thereby generate validation information, and incorporates said validation information into said encoded document; and wherein said decoder module includes a validation module which is responsive to said validation information to validate whether a data structure of said reproduced internal representation is in compliance with a predetermined document type declaration.