该代码与flexible.js原理是一样的，本人已使用该js做过n个移动端页面了。

(function(e,t){function n(){for(var e=navigator.userAgent,t=["Android","iPhone","SymbianOS","Windows Phone","iPad","iPod"],n=!0,i=0;t.length>i;i++)if(e.indexOf(t[i])>0){n=!1;break}return n}function i(){var t=a.getBoundingClientRect().width;n()&&2047>t&&(t=640);var i=t*100/w;a.style.fontSize=i+"px",d.rem=e.rem=i}var r,o=e.document,a=o.documentElement,s=o.querySelector('meta[name="viewport"]'),c=o.querySelector('meta[name="flexible"]'),l=0,u=0,d=t.flexible||(t.flexible={});var w=o.querySelector('meta[name="W_design"]')?o.querySelector('meta[name="W_design"]').getAttribute('content'):640;if(s){var p=s.getAttribute("content").match(/initial\-scale=([\d\.]+)/);p&&(u=parseFloat(p[1]),l=parseInt(1/u))}else if(c){var f=c.getAttribute("content");if(f){var h=f.match(/initial\-dpr=([\d\.]+)/),m=f.match(/maximum\-dpr=([\d\.]+)/);h&&(l=parseFloat(h[1]),u=parseFloat((1/l).toFixed(2))),m&&(l=parseFloat(m[1]),u=parseFloat((1/l).toFixed(2)))}}if(!l&&!u){e.navigator.appVersion.match(/android/gi);var g=e.navigator.appVersion.match(/iphone/gi);e.navigator.appVersion.match(/ipad/gi);var v=e.devicePixelRatio;l=g?v>=3&&(!l||l>=3)?3:v>=2&&(!l||l>=2)?2:1:1,u=1/l}if(a.setAttribute("data-dpr",l),!s)if(s=o.createElement("meta"),s.setAttribute("name","viewport"),s.setAttribute("content","initial-scale="+u+", maximum-scale="+u+", minimum-scale="+u+", user-scalable=no"),a.firstElementChild)a.firstElementChild.appendChild(s);else{var y=o.createElement("div");y.appendChild(s),o.write(y.innerHTML)}e.addEventListener("resize",function(){clearTimeout(r),r=setTimeout(i,300)},!1),e.addEventListener("pageshow",function(e){e.persisted&&(clearTimeout(r),r=setTimeout(i,300))},!1),"complete"===o.readyState?o.body.style.fontSize=12*l+"px":o.addEventListener("DOMContentLoaded",function(){o.body.style.fontSize=12*l+"px"},!1),i(),d.dpr=e.dpr=l,d.refreshRem=i,d.rem2px=function(e){var t=parseFloat(e)*this.rem;return"string"==typeof e&&e.match(/rem$/)&&(t+="px"),t},d.px2rem=function(e){var t=parseFloat(e)/this.rem;return"string"==typeof e&&e.match(/px$/)&&(t+="rem"),t}})(window,window.lib||(window.lib={}));

使用方法：

如果元素尺寸为50px，则为0.5rem，100px为1rem，以此类推...

该js默认设计稿宽度为640，如果宽度为750，需要在头部加一段代码

<meta name="W_design" content="750" />