var num = 1 / window.devicePixelRatio;var fontSize = document.documentElement.clientWidth / 10;document.getElementsByTagName('html')[0].style.fontSize = fontSize + 'px';

适配移动端rem单位，实际使用的时候用量取到的像素值/75即为计算后的rem值，标准为iPhone6的750的尺寸。750px的设计稿的标准宽度为10rem；

补充另一种，本质不变

!function (a, d) {    function c() {        var t = document.documentElement.clientWidth,            i = document.documentElement.clientHeight;        d.documentElement.style.fontSize = ((t / i) >= 0.75) ? t * 0.1 + "px" : t / 750 * 75 + "px"    };    var b = null;    a.addEventListener("onorientationchange" in a ? "orientationchange" : "resize", function () {        clearTimeout(b);        b = setTimeout(c, 300)    }, !1);    a.addEventListener("pageshow", function (a) {        a.persisted && (clearTimeout(b), b = setTimeout(c, 300))    }, !1);    d.addEventListener("DOMContentLoaded", c, false)}(window, document);