It seems that the "empty" step should run even if the UA is not processing the
iframes' attributes for the first time; see
Posted from: 2001:4c28:a030:30:c15f:2afb:ede8:7db0
User agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:17.0) Gecko/17.0 Firefox/17.0
That test shows that the step _shouldn't_ run, actually, but that we should navigate to about:blank with replacement enabled.
Looks like it should be "with replacement enabled" if the page has not "completely loaded", much as for form submission or the location.href setter.
Checked in as WHATWG revision r7378.
Check-in comment: Attempt to make <iframe> loading more closely match reality. See tests in bug.