js动态计算移动端rem适配

第一：css3的media query来实现适配，例以下面这样：

html {
    font-size : 20px;
}
@media only screen and (min-width: 401px){
    html {
        font-size: 25px !important;
    }
}
@media only screen and (min-width: 428px){
    html {
        font-size: 26.75px !important;
    }
}
@media only screen and (min-width: 481px){
    html {
        font-size: 30px !important;
    }
}
@media only screen and (min-width: 569px){
    html {
        font-size: 35px !important;
    }
}
@media only screen and (min-width: 641px){
    html {
        font-size: 40px !important;
    }
}

第二经过js动态设置html字体，例以下面这样

var docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
        recalc = function () {
            var clientWidth = docEl.clientWidth;
            if (!clientWidth) return;
           // 默认设计图为640的状况下1rem=100px；根据本身需求修改
            if(clientWidth>=640){
                docEl.style.fontSize = '100px';
            }else{
                docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
            }
        };

    if (!doc.addEventListener) return;
    win.addEventListener(resizeEvt, recalc, false);
    doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);