If you want to find out if a YouTube-video (that you don't own) can be embedded in an iframe, you should not rely on what you see on your hard disk, because something like
<iframe src="https://www.youtube.com/embed/VIDEO-ID"></iframe> may not work locally (message: 'video not available') while working fine on the internet. Putting
https://www.youtube.com/embed/VIDEO-ID directly in the address bar may also give you incorrect information as to the 'embeddability' of the video. The only sure way to know whether or not a YouTube-video can be embedded in an iframe is to see what happens if you put
<iframe src="https://www.youtube.com/embed/VIDEO-ID"></iframe> online. If the video plays, everything is okay. If it refuses to play, that means that the video can't be embedded.
But this is a time-consuming procedure, so I made a page (this one) that puts
<iframe src="https://www.youtube.com/embed/VIDEO-ID"></iframe> online for you. The only thing you have to do is to paste the ID of any YouTube-video in the input field under the empty iframe below and then click on 'Check'. You'll instantly know whether or not the video is available for embedding.