var calculatSeize = function () {
	var BASE_FONT_SIZE = 100;

	var docEl = document.documentElement,
	    clientWidth = docEl.clientWidth;
	if (!clientWidth) return;
	docEl.style.fontSize = BASE_FONT_SIZE * (clientWidth / 720) + 'px';

};

// Abort if browser does not support addEventListener
if (document.addEventListener) {
    var resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize';
    window.addEventListener(resizeEvt, calculatSeize, false);
    document.addEventListener('DOMContentLoaded', calculatSeize, false);
    calculatSeize();
}