CSS for "fill parent?"

Depending on what you inner item is, there are various approaches.

If it's a block-level element (a paragraph, a div, etc.), it will automatically adjust itself to fill 100% of the container's width.

If it's an inline element, too bad for you, it won't accept width:100% until you convert it to a block-level element: display:block.

Floated elements are a special case: they will only span to the width of their inner content, even if they're block level elements. They require width:100%.

Absolutely positioned elements are even tougher: they need width:100%, but the container also needs a positioning context, eg. position:relative.

Examples of all four cases: http://jsfiddle.net/dD7E4/

Have you tried: width: 100%; ?