How can I implement a search button inside the search box, as seen on a site like Bing?

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It just looks like it’s inside but it’s not (you cannot put html inside an input).
The 2 elements (an input and a button) are close together with 0 margin and both have the same height. The button’s graphic has a 3px white margin. So it creates this effect.
A possible markup and styling could be: