It seems that embedded youtube videos can not be currently be center aligned. 

There is also a bug when you embed a youtube video in a table, then cut and paste the table.
The video will suddenly take up the entire post, however large the browser window may be, making it very difficult to repair.
Youtube videos also can not be center aligned within a table.

Hope this helps

 

That is exactly same as my issue. It cannot be center aligned within a table. Help

I have reported this issue in the past to ips, there is no solution for this.

Up this topic. This stay actually. Our users can't create some news and guides and set youtube embedded to center align

Same issue for me, started with 4.2 upgrade. If I center align a Youtube video, it previews as being full width then when I submit the post it condenses it to link only.