Expand a div to fill the remaining width

Question

I want a two-column div layout, where each one can have variable width e.g.

Answer 1

I just discovered the magic of flex boxes (display: flex). Try this:

 
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Flex boxes give me the control I've wished css had for 15 years. Its finally here! More info: https://css-tricks.com/snippets/css/a-guide-to-flexbox/