designSize=640 为设计稿大小
htmlFontSize=100为当设计稿为640px时html font-size为100px
(建议默认100，因为好换算,10也可以，但pc有些浏览器会不支持12px以下字体，所用100最安全)
此时1px=0.01rem;
(function(doc, win, designSize,htmlFontSize) {
var docEl = doc.documentElement,
isIOS = navigator.userAgent.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/),
dpr = isIOS ? Math.min(win.devicePixelRatio, 3) : 1,
dpr = window.top === window.self ? dpr : 1, //被iframe引用时，禁止缩放
resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize';
docEl.dataset.dpr = dpr;
var recalc = function() {
var width = docEl.clientWidth;
if (width / dpr > designSize) {
width = designSize * dpr;
}
docEl.dataset.width = width;
docEl.dataset.percent = htmlFontSize * (width / designSize);
docEl.style.fontSize = htmlFontSize * (width / designSize) + 'px';
};
recalc();
if (!doc.addEventListener) return;
win.addEventListener(resizeEvt, recalc, false);
})(document, window,640,100);
```
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