JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box1" class="equals">content here</div>
<div id="box2" class="equals">content here<br />yes</div>
<div id="box3" class="equals">content here</div>

CSS

div.equals {
    background: red;
    float: left;
    width: 200px;
    margin: 5px;
}

JavaScript

Elements.implement({
    setEqualHeight: function() {
        this.setStyle("height",  Math.max.apply(Math, this.map(function(el) { return el.getSize().y })));
    } 
});

document.getElements("div.equals").setEqualHeight();