content: If you disable 'Dash' on youtube, they are often stored on your HD by default. In firefox based browsers by toggling the about:config setting "media.mediasource.enabled" to false, it disables 'Dash' and allows the video to cache so that you can rewind the video without it having to re-download parts. Also, one could always just "save page as" and the mp4 video file will be in the saved file.