Annotations are a broadly useful mechanism that can support a number of useful document management applications (third-party commentary, design rationale, information filtering, and semantic labeling of document content to name a few). The ubiquity of web content motivates the need for web annotation systems that are lightweight, efficient, non-intrusive (preferably transparent), platform-independent and scaleable. Building such a system using open and standard web infrastructures (as opposed to proprietary ones) facilitates widespread applicability and deployment. In practice, there are a number of ways to do this, all of which instantiated a common abstract architecture based on intermediaries. The paper describes our experiences with client and proxy-server based implementations of the annotation system architecture. The implementations point to missing elements in the current web infrastructure that make any implementation of annotation systems less than completely satisfactory. This paper discusses these elements of current web infrastructure, and potential changes to the web architecture that might make the implementation of annotation systems more complete.
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