JSFiddle - React, Tailwind, and code Playground

HTML

<div id="Container">
           <div id="Header">HEADER</div>
            <div id="Content">
                <div id="Body">
                    <div class="one_third box_dark"> ONE </div>
                    <div class="one_third box_medium"> TWO </div>
                    <div class="one_third last box_light"> THREE </div>
                    <p>Fusce auctor orci sit amet nibh. Curabitur tellus nisl, varius in, interdum quis, dapibus vitae, ligula. Nam lobortis fermentum ante. Phasellus ligula. Pellentesque iaculis iaculis lectus. Aenean sagittis, lacus eget imperdiet consectetuer, wisi dolor ornare velit, non scelerisque purus orci sit amet metus. Donec tincidunt est vitae metus. Integer porta leo sit amet quam mollis molestie. Quisque at magna ut est congue pulvinar. In ullamcorper fermentum nunc. Aenean viverra libero id libero lobortis tincidunt. Ut lorem velit, mattis in, varius id, vulputate nec, dolor. Sed justo dolor, luctus ut, feugiat ut, nonummy sit amet, erat. Sed ullamcorper elit ut turpis.</p>
                </div>
            </div>
            <div id="Footer">FOOTER</div>
        </div>

CSS

#Container {
        width: 960px;
        margin: 0 auto;
    }
    #Content {
        margin-top: 1em;
    }
    #Body {
        margin-bottom: 1em;
        overflow: auto;
    }
    #Body p { /* since this is not supported natively by early versions of IE */
        clear: both; /* how do I avoid having to do this ? */
        border: 1px solid red;
    }
    .one_third { width:29.66%; }
    .two_third { width:64.33%; }
    .one_third, .two_third { position:relative; margin-right:3%; margin-bottom: 0.66em; float:left; min-height: 4em; padding: 0.33em; border: 3px solid rgb(209,221,221); border-radius: 12px; }
    .last { margin-right:0 !important; clear:right; }
    .box {color: black; background-color: rgb(209,221,221); /* blue-gray */}
    .box_dark {color: white; background-color: rgb(0,40,72); /* darkblue */}
    .box_medium {color: white; background-color: rgb(1,57,106); /* mediumblue */}
    .box_light {color: white; background-color: rgb(54,94,155); /*lightblue */}