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dabblet.css

* {
	box-sizing:border-box; /* www.paulirish.com/2012/box-sizing-border-box-ftw/ */‎
	}

#all-wrapper {
    width: 960px;
    margin: 0 auto;
	}

#content-left {
    float: left;
    width: 180px;
    min-height: 400px;
    background: indianred;
	}

#content-middle {
    display: block;
    overflow: hidden; /* www.stubbornella.org/content/2009/07/23/overflow-a-secret-benefit/ */
    
    min-height: 300px; /* demo purposes */
    background: dodgerblue;
    border: 5px dashed;
	}
	
#content-right {
    float: right;
    min-width: 280px;/* change the value and see the middle box adjust its width to take available space*/
    min-height: 200px;
    background: green;
	}

dabblet.html

<div id='all-wrapper'>
	<div id="content-area" class="clearfix">
   		<div id="content-left"></div>
    	<div id="content-right"></div>
    	<div id="content-middle"></div>
    </div>
</div>

dabblet.js

// alert('Hello world!');

settings.json

{"view":"separate","fontsize":"100","seethrough":"","prefixfree":"1","page":"all"}