You pretty much have the same issue with any desktop software - you have to trust the authors and the server that gives you the download files (or the server(s)

You pretty much have the same issue with any desktop software - you have to trust the authors and the server that gives you the download files (or the server(s) that give you checksums and public keys to verify the download). Doing in-browser crypto requires less trust than server-side crypto. With in-browser crypto, the server would have to do an active attack and serve malicious scripts, which could be detected. When everything happens in the server all the time, it would be a passive attack that doesn't require any client-side modification and could easily go undetected. Also, with open-source websites, people could audit the source code and ensure it works as advertised. Combined with a browser extension that verifies the source code from the website matches the code on the public code repository, this could make it much easier to trust websites to do in-browser crypto right.