The W3C Semantic Web Annotea Advanced Development project has been working on using the Annotea framework to implement shared bookmarks. Annotea bookmarks (which like Annotea annotations are RDF statements) provide the usual functions associated with browser bookmarks and in addition let users add other RDF properties to them, share them with other users or between browsers, and query the bookmark data in new ways not supported by current bookmark systems.

Bookmarks are classified by Topics which are themselves described using RDF properties and may be arranged in strict hierarchies or in more complex graphs according to the users' needs. Users share bookmarks by accessing a common RDF data store and referring to common URIs for the names of their topics.

Annotea bookmarks being RDF statements, a variety of user interfaces are both possible and desirable. A simple UI can be implemented in browsers while leaving richer manipulations to other applications, such as visual editors for RDF like IsaViz.

IsaViz represents models as editable node-link diagrams, making the graph structure easier to understand and navigate compared to textual serializations such as RDF/XML. However, such diagrams can quickly become big and over-cluttered. Features such as zoomable user interfaces and overview windows, featured in IsaViz 1, offer enhanced navigation capabilities and are a first (partial) solution to this problem. Another complementary solution consists in further taking advantage of the properties associated with visual representations to offer better and easier-to-understand diagrams.

IsaViz 2 features a mechanism called graph stylesheets which makes it possible to add style to RDF models represented as standard node-link diagrams, based on resource and literal typing, property names, and additional information contained in schemas. This mechanism allows one to specify alternate layouts, filter information and use more perceptual dimensions to encode information visually, thus reducing the number of entities in the graphical representation while at the same time increasing the amount of parallelism in the user's cognitive processing of the model.

Graph stylesheets can be designed and customized for any RDF application. We will here illustrate their use on Annotea bookmarks.