rem 的使用

1.填加以下代码

(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;}';
    // console.log('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;
  }

  refreshRem(750);

  win.addEventListener("resize", function () {
    clearTimeout(tid);
    tid = setTimeout(refreshRem, 300);
  }, false);

  win.addEventListener("pageshow", function (e) {
    if (e.persisted) {
      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, 1024);

　　
补充: em相对于父元素，rem相对于根元素。

简单版

(function () {

var docEl = document.documentElement;

var resize = 'orientationchange' in window ? 'orientationchange' : 'resize';

var setRem = function () {

var screenWidth = docEl.clientWidth || window.screen.width || 375;

// 1080 PSD宽度(可变的)

docEl.style.fontSize = (100 * screenWidth / 375) + 'px';

};

window.addEventListener('resize', setRem, false);

setRem();

})(); // 用法psd量出来的像素距离 除以100 比如psd: 100px 转换后 1rem;