:hover lost over margin in IE8 & 10

HTML

<div class="a">
<div class="b">
  123
</div>
</div>

CSS

.a {
  width: 200px;
  height: 200px;
  background: grey;
  display: block;
  margin: 0;
}

.b {
  width: 50px;
  height: 50px;
  background: red;
   margin: 40px;
   display: block;
  
}

.b:hover {
  background: green;
}