Abstract: A web content extension system and platform is described. Web content extensions include tags as well as actionable code that supports deeper integration with web content than that supported by conventional tags, e.g., through interaction with APIs of the web content. The web content extensions are made available by developers of the extension to content creators via an extension catalog. The extension catalog thus supports a platform that unites developers of the web content extensions with content creators of web content. Further, this platform may be configured to support developer control of integration of the web content extension through use of an extension user interface that is created by the developer.