A network client such as a world wide web browser and corresponding method that includes and involves a scanner component that accesses an input content stream via a network connection (e.g., file, document or other content/data source located at a URL via the Internet and www., etc.) to extract renderable content therefrom, a parsing component coupled to the scanner component for parsing the renderable content, and a replaceable document type definition component configured to control the parsing component based on a particular document type definition corresponding to a particular grammar. The replaceable document type definition component being replaceable during execution of the network client. The network client and its corresponding method may be used within a data processing system to receive and manifest content based on a document type definition that is not otherwise known prior to execution and run time of the network client.