Officially confirmed to be impossible.
Answer from GitHub support:
Thanks for reaching out to GitHub Support about using GitHub Pages
We set the following Cache-Control header for all GitHub Pages content:
This header is the same for all assets on all sites on our Pages service, and we don't currently provide a way to alter the ...
I came up with some solutions and they seem to work at the moment:
First, the easiest one. Use the bookmarklet I have mentioned here. When you click the bookmark, YouTube video will stop.
If you need a button, use this userscript.
I recently created my own Google Chrome Extension. If you are a Chrome user, you can make use of this extension instead of the ...
At this time the GMAIL web UI can't. The alternatives are to use a POP or IMAP client.
Gmail Help Forum
Turning conversation view OFF causing another problem: I can't see my replies in email threads.
Looks like I’m a bit late. As Sk Nepal mentioned, you can use a simple bookmarklet to address the issue. It will be workable as long as YouTube has a static player ID.
Here is my version of the bookmarklet.
The only way to stop your download is to click at the end of your YouTube video.
If you are looking for the thumb that the video use: press Ctrl + U.
Search for this line:
meta property="og:image" content="http://anurl.com/vi/letters.jpg?feature=og"
You can use Ctrl + F for searching this line. Copy and paste the URL.