var calculate_size = function () {
    var BASE_FONT_SIZE = 100, docEl = document.documentElement, clientWidth = docEl.clientWidth;
    if (clientWidth) {
        docEl.style.fontSize = BASE_FONT_SIZE * (clientWidth / 750) + 'px';
    }
};
// Abort if browser does not support addEventListener
if (document.addEventListener) {
    var resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize';
    window.addEventListener(resizeEvt, calculate_size, false);
    document.addEventListener('DOMContentLoaded', calculate_size, false);
    calculate_size();
}