code :
<style>.a{height:24px; overflow:hidden;font-size:12px;}</style>
<div style="width:283px; border:1px sodivd blue;">
  <div style="background:yellow;float:left;height:80px;width:270px;
opacity:.2;filter:alpha(opacity=20)"></div>
  <div style="width:250px; background:greenyellow">
      <div class="a" style="background:gray;">a a a a</div>
      <div class="a"  style="background:blue;">b b b b</div>
      <div class="a" style="background:orange;">c c c c</div>
    </div>
</div>

some rule about float and overflow:
rule 1:
    Since a float is not in the flow, non-positioned block boxes created
before and after the float box flow vertically as if the float did
not exist. However, line boxes created next to the float are shortened
to make room for the margin box of the float. If a shortened line box is
too small to contain any further content, then it is shifted downward
until either it fits or there are no more floats present. Any content in
the current line before a floated box is reflowed in the first
available line on the other side of the float. In other words, if inline
boxes are placed on the line before a left float is encountered that
fits in the remaining line box space, the left float is placed on that
line, aligned with the top of the line box, and then the inline boxes
already on the line are moved accordingly to the right of the float (the
right being the other side of the left float) and vice versa for rtl
and right floats.

rule 2:
    The border box of a table, a block-level replaced element, or an
element in the normal flow that establishes a new block formatting
context (such as an element with 'overflow' other than 'visible') must
not overlap any floats in the same block formatting context as the
element itself. If necessary, implementations should clear the said
element by placing it below any preceding floats, but may place it
adjacent to such floats if there is sufficient space. They may even make
the border box of said element narrower than defined by section 10.3.3.
CSS2 does not define when a UA may put said element next to the float
or by how much said element may become narrower.

problem:
there're different performance in IE, webkit and gecko.
my god!
--
FROM 221.221.232.*