For embedded YouTube videos, can the player be 16:9 instead of 4:3? Most video game videos will be in this aspect ratio nowadays. And it takes no additional screen real-estate: if you just make the video player as wide as the answer area, and leave the height the same, you'll have a 16:9 player. And 4:3 video will just have black columns on the sides. (Example of an answer with embedded YouTube video.)
How it is today
How it could be
Note: I requested this some time ago over on Meta Stack Overflow and it got heavily downvoted (9 downvotes in a very short time). I still don't understand why anyone wouldn't want this, so I'm asking here in the hopes that we can at least get this on the Gaming site.
If anyone is interested, I wrote a GreaseMonkey script to do this that's over on StackApps.