I added overflow:hidden;
and height:20px;
to the parent div. Now it works fine.
See the jsFiddle
The CSS now looks like this:
.progress {
overflow: hidden;
width:100px;
height:20px;
}
.progressleft {
float: left;
height: 20px;
background: #6d6
}
.progressright {
overflow: hidden;
height: 20px;
background: #d66;
}