Borders have three main pieces: a width, a style, and a color; the style is required for any of the others to work.
Try adding the style:
border-style: solid;
Also, you can specify all these in the same line of css:
border: thin solid black;
Updated
As pointed out by Wesley, border-style is the only required one.
From http://www.w3schools.com/css/css_border.asp :
None of the border properties will have ANY effect unless the border-style property is set!
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