根据设计稿的实际宽度值,与设计稿最大宽度值,动态计算根字体大小，适应不同的屏幕比例。  

本文的px转rem换算公式为:

100px = 1rem 
18px = 0.18rem

基础css

body,dl,dd,ul,ol,h1,h2,h3,h4,h5,h6,pre,form,input,textarea,p,hr,thead,tbody,tfoot,th,td{margin:0;padding:0;}        
ul,ol{list-style:none;}
a{text-decoration:none;}
html{-ms-text-size-adjust:none;-webkit-text-size-adjust:none;text-size-adjust:none;}
body{line-height:1.5; font-size:14px;}
body,button,input,select,textarea{font-family:'helvetica neue',tahoma,'hiragino sans gb',stheiti,'wenquanyi micro hei',\5FAE\8F6F\96C5\9ED1,\5B8B\4F53,sans-serif;}
b,strong{font-weight:bold;}
i,em{font-style:normal;}
table{border-collapse:collapse;border-spacing:0;}
table th,table td{border:1px solid #ddd;padding:5px;}
table th{font-weight:inherit;border-bottom-width:2px;border-bottom-color:#ccc;}
img{border:0 none;width:auto\9;max-width:100%;vertical-align:top; height:auto;}
button,input,select,textarea{font-family:inherit;font-size:100%;margin:0;vertical-align:baseline;}
button,html input[type="button"],input[type="reset"],input[type="submit"]{-webkit-appearance:button;cursor:pointer;}
button[disabled],input[disabled]{cursor:default;}
input[type="checkbox"],input[type="radio"]{box-sizing:border-box;padding:0;}
input[type="search"]{-webkit-appearance:textfield;-moz-box-sizing:content-box;-webkit-box-sizing:content-box;box-sizing:content-box;}
input[type="search"]::-webkit-search-decoration{-webkit-appearance:none;}
input:focus{outline:none;}
select[size],select[multiple],select[size][multiple]{border:1px solid #AAA;padding:0;}
article,aside,details,figcaption,figure,footer,header,hgroup,main,nav,section,summary{display:block;}
audio,canvas,video,progress{display:inline-block;}
body{background:#fff;}

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js

脚本计算 rem单位 需要的基准字体大小  

//designWidth:设计稿的实际宽度值，需要根据实际设置
var DesignWidth = 375;
//maxWidth:制作稿的最大宽度值，需要根据实际设置
var MaxWidth = 375;
//这段js的最后面有两个参数记得要设置，一个为设计稿实际宽度，一个为制作稿最大宽度，例如设计稿为750，最大宽度为750，则为(750,750)
(function(designWidth, maxWidth) {
  var doc = document,
      win = window,
      docEl = doc.documentElement,
      remStyle = document.createElement("style"),
      tid;

  function refreshRem() {
    var width = docEl.getBoundingClientRect().width;
    maxWidth = maxWidth || 540;
    width>maxWidth && (width=maxWidth);
    var rem = width * 100 / designWidth;
    remStyle.innerHTML = 'html{font-size:' + rem + 'px;}';
  }

  if (docEl.firstElementChild) {
    docEl.firstElementChild.appendChild(remStyle);
  } else {
    var wrap = doc.createElement("div");
    wrap.appendChild(remStyle);
    doc.write(wrap.innerHTML);
    wrap = null;
  }
  //要等 wiewport 设置好后才能执行 refreshRem，不然 refreshRem 会执行2次；
  refreshRem();

  win.addEventListener("resize", function() {
    clearTimeout(tid); //防止执行两次
    tid = setTimeout(refreshRem, 300);
  }, false);

  win.addEventListener("pageshow", function(e) {
    if (e.persisted) { // 浏览器后退的时候重新计算
      clearTimeout(tid);
      tid = setTimeout(refreshRem, 300);
    }
  }, false);

  if (doc.readyState === "complete") {
    doc.body.style.fontSize = "16px";
  } else {
    doc.addEventListener("DOMContentLoaded", function(e) {
      doc.body.style.fontSize = "16px";
    }, false);
  }
})(DesignWidth, MaxWidth);