This runs entirely on the client, asside from one http request to a node server using the api from https://github.com/maple3142/ytdl. Simply fork the project and run a static file server.
I have used many different youtube downloaders in the past, but after hearing stories about malware being used on the sites / in the files (For example), and having several annoying ads shown to me, I decided to try to make my own. Its not very complicated (making it one of the fastest converters) and is completely open source.