html css: how to draw dotted box around <div>?

I saw in SO there is a lot of DIV which is开发者_StackOverflow surrounded by dotted lines, how to do it in CSS & HTML?

Thanks. Bin

<div style="border: dotted 1px black" id="mydiv">content</div> 

or

<style type='text/css'>
    #mydiv { border: dotted 1px black; }
</style>