iframe without an src attribute

I would like to create an <iframe> on the page, but then add the src later. If I make an iframe without an src attribute, then it loads the current page in some browsers. What is the correct value to set for the src so that it just loads a blank iframe?

The answers I've seen are:

• about:blank
• javascript:false
• javascript:void(0)
• javascript:"";
• url to a blank page

Is there a clear winner? If not, what are the tradeoffs?

I'd like to not have mixed content warnings for HTTPS urls, nor any back-button, history, or reload weirdness in all browsers from IE6 onward.