IFrames fail to work for me.
 

The <iframe> tag fails to work in my posts whenever I try to use it, making it impossible for me to embed YouTube videos and whatever else the tag allows one to do. This doesn't seem to be the case for other users and I don't see any reason why I would happen to be the person it doesn't work for. If there's anything anybody can do about this or if you could suggest a solution if the problem is on my side, I would be thankful.

It's an XSS thing, I think.

Well, if it were a security issue as you suggest, wouldn't others be equally affected by it? Instead, as I mentioned, everybody else can post iframes just correctly. My attempts to do so result in an empty space of specified size. In fact, it doesn't work even when I do as little as quote somebody else's iframe.
Edit: Well fuck me it works.
It still doesn't seem to show up correctly on previews though, and I swear it didn't work in a post last time I tried it.

You have also same problem like me that shown the large blank when check the previews.(well just my thought, it was my personal problem..)

Anyway, good to hear that your problem is now solved. ;)