Today's critical systems increasingly rely on computers and software. However, market pressure, problems in the application of formal methods, and ineffective traceability techniques may all exacerbate the difficulty of applying adequate assurance techniques to the design and development of safe and trustworthy systems. Necessity dictates that engineers target critical sections that require formal verification and high reliability. To achieve this objective, we need to implement and maintain linking relationships among system work products and be able to propagate criticality of requirements to subsequent work products. We successfully implemented traceability between an informal requirements document and its formal specification using two new XML-derived markup languages. We addressed the issues of specifying and propagating criticality of requirements and consistency of this determination within and between work products.