rem

(function (doc, win) {
            var docEl = doc.documentElement,
                    resizeEvt = 'onorientationchange' in window ? 'onorientationchange' : '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;
            win.addEventListener(resizeEvt, recalc, false);
            doc.addEventListener('DOMContentLoaded', recalc, false);
        })(document, window);