(function fn(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 * 75) / designwidth;
    // (basewidth / designwidth) * screenwidth
    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
    );
  }
})(750, 750);