As the web shifts towards mobile devices without support for Adobe Flash, developers need to use new technologies to bring the type of animated, interactive content they used to develop in Flash to those mobile devices. In this thesis, we present the design and implementation of a tool that eases this transition by automatically converting Flash to HTML5. We propose a new type of transformation using a server-side compiler and client-side interpreter that benefits the performance, file size overhead and interoperability of the conversion. The converter, named Swiffy, is evaluated on a dataset of Flash advertisements by measuring the percentage of files that are fully supported, the accuracy of the conversion and the performance of the output on desktop browsers and mobile devices. Swiffy provides Flash to HTML5 conversion in Google AdWords and is available for anyone to use as an extension to Adobe Flash Professional or using an online conversion service. Since the public release, millions of SWF files have been converted and Swiffy is now used across the Internet.