Hmm. I don't know that I've seen the float property used like that; you might try this instead (what I take you're going for) -
Code:
<div align="left">abc</div>
<div style="border:1px solid #000000";>def</div>
If I'm not mistaken, the float property makes the element's position 'absolute' for lack of a better word. It's static in its positioning, in other words. Other elements are supposed to flow around it, although another <div> tag may not (which is what we're seeing here)...