I\'d like to get the aspect ratio of a YouTube video, to resize the player accordingly. I\'m programming the YT player using JavaScript.
Maybe not a good answer, but there seems to be an assumption amongst other answers that YouTube videos are either 16:9 or 4:3.
But they can have a pretty much arbitrary aspect ratio, and with portrait phone videos having become quite common, it's becoming less of a rarity for a video on YouTube to be something different.
For these non-standard aspect ratios, as a quick manual fudge, I've resorted to playing them in full screen, doing a screen capture, and cropping the image down.
I've put a couple of examples of arbitrary aspect videos at http://youtube-aspect-ratios.xtra.ink.