Click the 'plus' button to make the link appear.
If the video has already been processed by YoutubeInMp3, the link immediately starts a download. Else it leads to a page where you have to click "convert and download" to get the file. Hence I recommend middle-clicking on it to be sure it won't quit your video.
I made this for myself, the code is as simple as possible so you should not have any performance/compatibility issue. If you have any issue, please post a feedback (it's easy you don't even have to create a account if you sign in through google)
uses youtubeinmp3.com api http://youtubeinmp3.com/api/