JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/css/bootstrap-combined.min.css">
<script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.2/js/bootstrap.min.js"></script>
<body>
    <div class="somecontainer">
        <div class="container-fluid">
            <div class="row-fluid">
                <div class="span3">
                    sidebar content
                </div>
                <div class="span9">
                    cool content bro
                </div>
            </div>
        </div>
    </div>
</body>

CSS

body {
    padding: 10px; 
}
.span3 {outline: 1px solid red;}
.span9 {outline: 1px solid blue;}
.somecontainer {
    width: 50%;
    outline: 1px solid green;
}
.m .row-fluid > div.span3, .m .row-fluid > div.span9 {
    width: 100%;   
}

JavaScript

$(document).ready(function(){
    $('.somecontainer').on('resize',function(){
        if ($('.somecontainer').width() < 400) {
            $('.somecontainer').addClass('m');
        } else {
            $('.somecontainer').removeClass('m');
    });
});