1. A method, comprising: retrieving, from a plurality of sources, a plurality of files, each of which includes mark-up language code which, when interpreted by a browser, would cause the browser to render original page content for a page designed by a first party; receiving input that specifies, for each source of the plurality of sources, location information about where to inject executable code; after retrieving said plurality of files from said plurality of sources, determining, for a particular file of the plurality of files, a particular source from which the particular file was obtained; determining, based on the particular source, particular location information associated with the particular source; inserting, by a code injector tool under control of a second party that is different from said first party, said executable code into the particular file or in a document object model (DOM) generated based on the particular file, at one or more locations that are based on the particular location information associated with the particular source, to create a code-injected file; interpreting at least a portion of the code-injected file to produce generated content; wherein interpreting at least said portion includes executing at least some of the inserted executable code; wherein executing at least some of the inserted executable code includes executing inserted executable code that (a) modifies how executable code that originated in the file is executed; (b) excludes from the generated content a subset of the original page content; (c) generates new content that would not be in said original page content; or (d) simulates at least one user event; and providing at least a portion of the generated content to a particular application that performs operations based on the portion of the generated content, wherein the operations do not include displaying the portion of the generated content; wherein the method is performed by one or more computing devices.