css "left" not working

I have 2 divs, parent and child, I want that child left side (left border) will in center of parent.

Why this code not working? that is left: 50% for child, is not working.

<div id="outher">
    <div id="inner">

    </div>
</div>

css:

#outher {
   width: 1000px;
   height: 1000px;
   background-color: #ccc;
}

#inner {
   width: 400px;
   height: 300px;
   background-color: #090;
   left: 50%;
}

demo http://jsfiddle.net/vrse2/5/

You need to set position to absolute or relative:

#inner {
   width: 400px;
   height: 300px;
   background-color: #090;
   position: absolute;
   left: 50%;
}

CSS left only works with positioned elements.

Quoted from W3C

Values  <length> | <percentage> | auto | inherit
Initial value   auto
Applies to  positioned elements
Inherited   No

Try

#inner {
   width: 400px;
   height: 300px;
   background-color: #090;
   position: absolute;
   left: 50%;
}

Good read

1. MDN : CSS Reference -left (Best IMHO)
2. W3C : CSS/Properties/left