例子：
rem：相对长度单位。相对于根元素(即html元素)font-size计算值的倍数
! function (a) {
var j, b = a.document,
c = b.documentElement,
d = a.orientation || 0,
e = 640,
f = 9 / 16,
g = 100,
h = a.devicePixelRatio,
i = h ? 1 / h : 1;
b.write('<meta name="viewport" content="initial-scale=' + i + ",maximum-scale=" + i + ",minimum-scale=" + i +
',user-scalable=0">'), j = c.getBoundingClientRect().width, 90 == Math.abs(d) && (j *= f), c.style.fontSize = j /
h * g / (e / h) + "px"
}(window);
