Web applications are becoming increasingly popular. More and more types of multimedia data have been embedded into web pages for providing abundant information and user interface. However, these complex web pages could not be smoothly presented on devices without adequate functionalities or resource. An alternative approach is shifting tasks from resource-constrained device to surrogate with rich resource. In this paper, we present a heterogeneous auto-offloading framework based on web browser, and implement plug-in proxy to offload multimedia processing tasks to heterogeneous surrogate automatically. The plug-in proxy works transparently with existing browser, and does not require modifying, recompiling or relinking. We perform several plug-in proxies of Firefox by offloading multimedia data and execution logic to surrogate, and merging remote display seamlessly with web browser interface. The experimental results show that the proposed framework and plug-in proxies could enrich browser functionalities, reduce resource consumed by client, or improve performance of multimedia tasks.