rem.js

// rem.js

  (function (doc, win) {
    var docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
        recalc = function () {
            var clientWidth = docEl.clientWidth;
            if (!clientWidth) return;
                if (clientWidth >= 750) {
                    docEl.style.fontSize = '100px';
                } else {
                    docEl.style.fontSize = 100 * (clientWidth / 750) + 'px';
                }
            };
            if (!doc.addEventListener) return;
            recalc();
    })(document, window);