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Cascading Style Sheets have been introduced by the W3C as a mechanism for controlling the appearance of HTML documents. In this paper, we demonstrate how constraints provide a powerful unifying formalism for declaratively understanding and specifying style sheets for web documents. With constraints we can naturally and declaratively specify complex behavior such as inheritance of properties and cascading of conflicting style rules. We give a detailed description of a constraint-based style sheet model, CCSS, which is compatible with virtually all of the CSS 2.0 specification. It allows more flexible specification of layout, and also allows the designer to provide multiple layouts that better meet the desires of the user and environmental restrictions. We also describe a prototype extension of the Amaya browser that demonstrates the feasibility of CCSS.

Existing augmentations of web pages are mostly small cosmetic changes (e.g., removing ads) and minor addition of third-party content (e.g., product prices from competing sites). None leverages the structured data presented in web pages. This paper describes Sifter, a web browser extension that can augment a well-structured web site with advanced filtering and sorting functionality. These added features work inside the site's own pages, preserving the site's presentational style and the user's context. Sifter contains an algorithm that scrapes structured data out of well-structured web pages while usually requiring no user intervention. We tested Sifter on real web sites and real users and found that people could use Sifter to perform sophisticated queries and high-level analyses on sizable data collections on the Web. We propose that web sites can be similarly augmented with other sophisticated data-centric functionality, giving users new benefits over the existing Web.