Adapt Width to page size with CSS

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I am looking to use the same navigation (on top with Home, Health Library links) as seen here http://www.uillinoismedcenter.org/. In looking at the site's stylesheet, I see that the text links are one div and have a set width and the orangeish container is a div witha set width, both in pixels. I want my nav to resize with the page ( I am assuming both divs will have to resize). Is there a way to use css to adapt to page size?

[Chris]

use width: 100%;

[Percept]

or set no width at all.