Abstract : This document presents an excerpt from the research results that I have obtained since I received a PhD in December 2006. My research addresses one fundamental challenge of our time: building the necessary theoretical foundations and practical tools for ensuring guarantees on web applications such as robustness, security, privacy, and efficiency, towards a web of trust. For this purpose, I build static analysis methods that analyse the source code of web applications for the purpose of automatically detecting defects, or certifying guarantees such as the absence of certain kinds of errors, otherwise. The methods that I develop can also be used for performing semantically-verified optimisations, in compiler design for instance. One particularity of my approach resides in the introduction of novel reasoning techniques, based on new rich modal logics and innovative algorithmic techniques. Another particularity of my research is that it considers theoretical aspects, algorithmic aspects as well as applied aspects such as implementation techniques, experimental validation, and practical relevance. The overall goal of my research is to enable people to construct more reliable, secure, and efficient information systems.