JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://kelvinator.dk/css/bootstrap-responsive.css">
<link rel="stylesheet" href="http://kelvinator.dk/css/bootstrap.min.css">
<body>
    <div class="container">

        <div class="row-fluid">

            <div class="span6">
                <div class="alert alert-info">
                    this has text <br />
                    this has text <br />


                </div>
            </div>
            <div class="span6">
                <div class="alert alert-success">
                    <div class="media">
                        This needs to be same size as the other box.<br />
                        This needs to be same size as the other box.<br />  
                        This needs to be same size as the other box.<br />
                        This needs to be same size as the other box.<br />
                        This needs to be same size as the other box.<br />
                        This needs to be same size as the other box.

                    </div>
                </div>
            </div>
        </div>

    </div>



</body>

JavaScript

function setHeights(){
    //get all the elements that need to be equal height:
    var elements = document.getElementsByClassName('alert');
    //call the equaliseHeights prototype method
    elements.equaliseHeights();
}

/* Extend the HTMLCollection prototype */
HTMLCollection.prototype.equaliseHeights = function() {
    var maxHeight=0;
    //loop through the collection to find the height of the tallest element:
    for (var i = 0; i < this.length; i++) {         
        this[i].style.minHeight = 0; //reset min-height     
        var thisHeight = this[i].offsetHeight; //measure height
        maxHeight = (thisHeight>maxHeight)?thisHeight:maxHeight; //store the greatest height
    }
    //now set the min-height for all:
    for (var i = 0; i < this.length; i++) {        
        this[i].style.minHeight = maxHeight+"px"; //set min-height
    }
    return this;
};

/* on load */
(function waitForReady(){
    if (document.readyState === "complete") {
        
        setHeights();//on load, call method
        window.onresize = setHeights;//on resize, call method
        
    } else {
        setTimeout(waitForReady,10);
    }
})();