Why does padding work as a property, but not as a css?

In my application this works:

Actionscript Code:

<s:VGroup width="100%" paddingLeft="100">
... stuff ...
</s:VGroup>

This doesn't:

Actionscript Code:

<fx:Style>
        @namespace s "library://ns.adobe.com/flex/spark";
        @namespace mx "library://ns.adobe.com/flex/mx";
        .myStyle {
            paddingLeft: 100;
        }
    </fx:Style>
<s:VGroup width="100%" styleName="myStyle">
... stuff ...
</s:VGroup>

Why not?