This is a port of youtubedown from Perl to JavaScript so that it can run natively in the browser.

To use it, bookmark this link: " Video Download", and then open that link while viewing the page of the video you want to download.

I had hoped to make it work with a form like the one below. The following form works on some video sites but not YouTube or Vimeo, due to Access-Control-Allow-Origin being set on some of the underlying data URLs. For YouTube to work, this JavaScript code has to be injected into the YouTube domain's page, which is what that bookmarklet does.