Letting people define their own inline frames on a forum would just be asking for trouble. I imagine that is why the Admins decided to give people the ability to embed videos from a couple video sharing sites by making the software recognize those links and auto-inserting the required <iframe> code for those sites. It gives people the option to embed videos but doesn't open the door to crazy stuff that might be possible by allowing the use of an <iframe>.

The anchor tag won't work either. It's not too difficult to change the <a href="URL">TEXT</a> code into [link:URL|TEXT], but once again the DU software limits what one can link to by using the [link:|] tag and parsing the contents and inserting the required <a ...></a> html in a post.

It's been quite a while since I tested for allowed html tags here to check if they added any new ones for our use, but I was pretty confident that as of my last edit to this OP every html tag that works here was listed. They do allow inline style in [div] tags so formatting options are pretty flexible, but as far as embedding content and links - they seem to want to manage that to some degree in order to prevent people (like me) from doing crazy shit.

As far as sharing videos from dailymotion, you're limited to just posting the URL:

  http://www.dailymotion.com/video/xhalb0_waking-life_shortfilms

[div style="width:100%;height:1.3em;border-bottom:1px solid #999999;"]