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1.通过css3媒体查询设置font-size

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; 
    }
}

2.通过js设置font-size

<script>
!function (window) { //来源：http://www.ydui.org/flexible
    var dw = 750,
    	d = window.document,
        docEl = d.documentElement,
        re = 'orientationchange' in window ? 'orientationchange' : 'resize';
    var recalc = (function refreshRem () {
        var clientWidth = docEl.getBoundingClientRect().width;
        docEl.style.fontSize = Math.max(Math.min(20 * (clientWidth / dw), 11.2), 8.55) * 5 + 'px';
        /*说明： 8.55：小于320px不再缩小，11.2：大于420px不再放大, 17.067 ：大于640px不再放大*/
        return refreshRem;
    })();
    docEl.setAttribute('data-dpr', window.navigator.appVersion.match(/iphone/gi) ? window.devicePixelRatio : 1);/* 添加倍屏标识，安卓为1 */
    if (/iP(hone|od|ad)/.test(window.navigator.userAgent)) {
        d.documentElement.classList.add('ios'); /* 添加IOS标识 */
        if (parseInt(window.navigator.appVersion.match(/OS (\d+)_(\d+)_?(\d+)?/)[1], 10) >= 8) /* IOS8以上给html添加hairline样式，以便特殊处理 */
            d.documentElement.classList.add('hairline');
    }
    if (!d.addEventListener) return;
    window.addEventListener(re, recalc, false);
    d.addEventListener('domContentLoaded', recalc, false);
}(window);
</script>

该方法以设计图尺寸750px为基准。100px替换单位为0.1rem。