Closer inspection of the DOM shows there is in fact a slight difference.
Mine, based on what you got when clicking "embed" in YouTube a few years back. It still seems to be the same today (should have a class or inline max-width actually)
<iframe src="//www.youtube.com/embed/6h_GChgSv_A" allowfullscreen="" width="560" height="315" frameborder="0"><a href="http://www.youtube.com/watch?v=6h_GChgSv_A" target="_blank" class="new_win">http://www.youtube.com/watch?v=6h_GChgSv_A</a></iframe>
Elk (note the lack of www)
<iframe style="max-width: 98%; max-height: auto;" src="//youtube.com/embed/6h_GChgSv_A?rel=0&autoplay=1" allowfullscreen="" width="640px" height="385px" frameborder="0"></iframe>
That automatically redirects you
$ curl -v http://youtube.com/embed/6h_GChgSv_A
* Trying 194.78.0.167...
* Connected to youtube.com (194.78.0.167) port 80 (#0)
> GET /embed/6h_GChgSv_A HTTP/1.1
> Host: youtube.com
> User-Agent: curl/7.47.0
> Accept: */*
>
< HTTP/1.1 301 Moved Permanently
< Date: Thu, 14 Apr 2016 12:03:53 GMT
< Server: gwiseguy/2.0
< Location: http://www.youtube.com/embed/6h_GChgSv_A
< Content-Length: 0
< Content-Type: text/html
< X-XSS-Protection: 1; mode=block
<
* Connection #0 to host youtube.com left intact
I'll send a PR to fix this up, but a useless redirect is just a minor detail you shouldn't even notice.