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