[css3-flexbox] what's the rendering of floated content *inside of a span*?

One last float-handling question...

What's the expected behavior of content like this, with a floated 
element *inside* a to-be-wrapped run of inline stuff:
   <div style="display:flexbox">
     <span>I am a span<div style="float:left">floated div</div></span>
   </div>

My initial expectation is that the entire <span> gets wrapped in an 
anonymous block, and the floated div floats to the left of *that* block. 
  (like "item6.1" in the current Editor's Draft, but with the div 
floating to the left of its wrapper)

However, I could perhaps see the <div> first being taken out-of-flow 
(reparented to the flexbox, leaving a placeholder behind in the <span>) 
and being turned into its own flexbox item, or something wacky like 
that...  I'm not sure whether that or my initial expectation is more 
correct/sensible.

Thanks!
~Daniel