This paper presents ConScript, an client-side advice implementation for security, built on top of Internet Explorer 8. ConScript allows the hosting page to express fine-grained application-specific security policies that are enforced at runtime. In addition to presenting 17 widely-ranging security and reliability policies that ConScript enables, we also show how policies can be generated automatically through static analysis of server-side code or runtime analysis of client-side code. We also present a type system that helps ensure correctness of ConScript policies. To show the practicality of ConScript in a range of settings, we compare the overhead of ConScript enforcement and conclude that it is significantly lower than that of other systems proposed in the literature, both on micro-benchmarks as well as large, widely-used applications such as MSN, GMail, Google Maps, and Live Desktop.