Providing isolated extensibility for webpages with a unified manifest and omni-accessible platform script

To provide isolated extensibility for webpages, thereby enabling webpages to provide additional functionality supported by instructions whose execution is independent of execution of the instructions of the webpage, a unified manifest is utilized which comprises information regarding each of the extensions that can extend the functionality of the webpage, and a common extensibility platform script is invoked that provides the setup functionality to support such isolated extensibility. The extensibility platform script can be utilized by each extension thereby centralizing the relevant extension instantiation functionality. To provide for a unified manifest, a manifest associated with one extension can be copied into a manifest associated with another extension if the first extension indicates that it is capable of extending the second extension, or if the first extension is actually invoked by the second extension. Such a copy can occur when the extensions and their corresponding manifests are uploaded onto a server.