h5移动端rem使用750设计稿

function getFontSize() {
  var baseFontSize = 100;
  var baseWidth = 750;
  var minWidth = 320;
  var clientWidth = window.innerWidth;
  var innerWidth = Math.max(Math.min(clientWidth, baseWidth), minWidth);
  var rem = clientWidth / (baseWidth / baseFontSize);
  if (clientWidth > baseWidth) {
    rem = 100
  }
  if (clientWidth < minWidth) {
    rem = minWidth / (baseWidth / baseFontSize);
  }
  document.querySelector('html').style.fontSize = rem + 'px';
}
(function(doc, win) {
  getFontSize();
  window.onresize = function() {
    getFontSize();
  }
})(document, window);