Dynamically customizing a user interface for the aggregation of content

Enabling dynamic aggregation of content from a plurality of content providers. The invention stores references associated with specific display areas in a document for a particular data source. Responsive to a request for the document, the invention retrieves the references associated with the data source and dynamically inserts the references into the appropriate display area of the document. A client renders the document by obtaining the content associated with the references. In an embodiment, the invention includes a web service for cobranding a login user interface for a user authentication system. The web service places cobranding content provided by an affiliate content provider and sign-in user interface content provided by a user authentication service into different frames of a single web page.