<metaname="viewport"content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">

代码一：

window.onload = function(){
    /*720代表设计师给的设计稿的宽度，你的设计稿是多少，就写多少;100代表换算比例，这里写100是为了以后好算,比如，你测量的一个宽度是100px,就可以写为1rem,以及1px=0.01rem等等*/
    getRem(720,100)
};

window.onresize = function(){
    getRem(720,100)
};

function getRem(pwidth,prem){
    var html = document.getElementsByTagName("html")[0];
    var oWidth = document.body.clientWidth || document.documentElement.clientWidth;
    html.style.fontSize = oWidth/pwidth*prem + "px";
}

代码二： 小米官网的写法

!function(n){

    var  e=n.document,

        t=e.documentElement,

        i=720,

        d=i/100,

        o="orientationchange"in n?"orientationchange":"resize",

        a=function(){

            var n=t.clientWidth||320;n>720&&(n=720);

            t.style.fontSize=n/d+"px"

        };

        e.addEventListener&&(n.addEventListener(o,a,!1),e.addEventListener("DOMContentLoaded",a,!1))

}(window);

代码三：自适应代码

    (function (win) {
        var doc = win.document;
        var docEl = doc.documentElement;
        var tid;
        function refreshRem() {
            var width = docEl.getBoundingClientRect().width;
            if (width > 750) { // 最大宽度
                width = 750;
            }
            var rem = width / 3.75;
            docEl.style.fontSize = rem + 'px';
        }
        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);
        refreshRem();
    })(window);

function htmlSize(){
    var oHtml = document.documentElement;
    var aWidth = oHtml.getBoundingClientRect().width;
    if(aWidth < 750){
        oHtml.style.fontSize = aWidth / 15 + "px";
    }else{
        oHtml.style.fontSize = 750 / 15 + "px";
    }   
}
(function(){
    htmlSize();
})();
window.onresize = function(){
    htmlSize();
}