Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for generating modified web documents. In one aspect, a method includes retrieving a web document including markup language code and having references to resources external to the web document and parsing the retrieved web document to interpret the markup language code and identify references to resources external to the retrieved web document. Data relating to at least a portion of the resources external to the retrieved web document are retrieved, and a modified web document including the retrieved data is generated and stored for use in responding to a request for retrieval of content of the web document.