When it comes to supporting multiple platforms, browser extensions exist somewhere in between websites and mobile apps. One end of the spectrum is the website, which enjoys a “write-once-run-anywhere” developer experience. On the other end of the spectrum is the mobile app, which is written in a proprietary language and will only work on a specific operating system and a subset of device types. In this chapter, we will discuss what challenges and tradeoffs are involved when supporting an extension on multiple browsers.