1. A computer readable storage medium encoded with a computer program to be executed by a computer for extracting semantic information about a plurality of documents autonomously created by different sources and being accessible via a computer network, said computer readable storage medium comprising: a tokenizer for causing the computer to generate a set of tokens indicative of document object model (DOM) nodes associated with visual information in a displayed document image from one of the plurality of autonomously created documents; a grammar mechanism for causing the computer to derive a non-prescribed visual grammar from the set of tokens to represent a hidden syntax convention of a visual presentation; and a best-effort parser for causing the computer to apply the derived visual grammar to construct multiple parse trees that represent semantic structure of the document and interpret a maximum subset of the set of tokens, wherein said non-prescribed visual grammar is derived from a plurality of autonomously created or heterogeneous Web documents to represent the hidden syntax convention of the visual presentation common among the plurality of autonomously created or heterogeneous Web documents; and said grammar is a five tuple <Σ, N, s, Pd, Pf> where Σ is a set of terminal symbols, N is a set of nonterminal symbols, sεN is a start symbol, Pd is a set of production rules that represent visual patterns and Pf is a set of preference rules that represent pattern precedence.