How do I position a div after a div or at the bottom depending the window size? (image example)

I want a div to be below its sibling, if the browser window is big enough or to be at the bottom. The div1 doesn't have a static height of course.

This image is how I want to position div2 if the browser window i开发者_JAVA技巧s big

The following is how I want div2 to be positioned if the browser window is small.

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You're describing a "sticky footer".

• http://ryanfait.com/sticky-footer/
• http://www.cssstickyfooter.com/