;
(function(win, lib) {
var doc = win.document;
var docEl = doc.documentElement;
var metaEl = doc.querySelector('meta[name="viewport"]');
var flexibleEl = doc.querySelector('meta[name="flexible"]');
var dpr = 0;
var scale = 0;
var tid;
var flexible = lib.flexible || (lib.flexible = {});

if (metaEl) {
    var match = metaEl.getAttribute('content').match(/initial\-scale=([\d\.]+)/);
    if (match) {
        scale = parseFloat(match[1]);
        dpr = parseInt(1 / scale);
    }
} else if (flexibleEl) {
    var content = flexibleEl.getAttribute('content');
    if (content) {
        var initialDpr = content.match(/initial\-dpr=([\d\.]+)/);
        var maximumDpr = content.match(/maximum\-dpr=([\d\.]+)/);
        if (initialDpr) {
            dpr = parseFloat(initialDpr[1]);
            scale = parseFloat((1 / dpr).toFixed(2));
        }
        if (maximumDpr) {
            dpr = parseFloat(maximumDpr[1]);
            scale = parseFloat((1 / dpr).toFixed(2));
        }
    }
}

if (!dpr && !scale) {
    var isAndroid = win.navigator.appVersion.match(/android/gi);
    var isIPhone = win.navigator.appVersion.match(/iphone/gi);
    var devicePixelRatio = win.devicePixelRatio;
    if (isIPhone) {
        // iOS下，对于2和3的屏，用2倍的方案，其余的用1倍方案
        if (devicePixelRatio >= 3 && (!dpr || dpr >= 3)) {
            dpr = 3;
        } else if (devicePixelRatio >= 2 && (!dpr || dpr >= 2)) {
            dpr = 2;
        } else {
            dpr = 1;
        }
    } else {
        // 其他设备下，仍旧使用1倍的方案
        dpr = 1;
    }
    scale = 1 / dpr;
}

docEl.setAttribute('data-dpr', dpr);
if (!metaEl) {
    metaEl = doc.createElement('meta');
    metaEl.setAttribute('name', 'viewport');
    metaEl.setAttribute('content', 'initial-scale=' + scale + ', maximum-scale=' +                       
   scale + ', minimum-scale=' + scale + ', user-scalable=no');
   if (docEl.firstElementChild) {
        docEl.firstElementChild.appendChild(metaEl);
    } else {
        var wrap = doc.createElement('div');
        wrap.appendChild(metaEl);
        doc.write(wrap.innerHTML);
    }
}
function refreshRem() {
    var width = docEl.getBoundingClientRect().width
    // 适配平板
    if (width / dpr > 750) {
        width = 750 * dpr
    }
    var rem = 75 * (width / 750)
    docEl.style.fontSize = rem + 'px'
    flexible.rem = win.rem = rem;
}
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 = 12 * dpr + 'px';
} else {
    doc.addEventListener('DOMContentLoaded', function(e) {
    doc.body.style.fontSize = 12 * dpr + 'px';
}, false);
}


refreshRem();

flexible.dpr = win.dpr = dpr;
flexible.refreshRem = refreshRem;
flexible.rem2px = function(d) {
    var val = parseFloat(d) * this.rem;
    if (typeof d === 'string' && d.match(/rem$/)) {
        val += 'px';
    }
    return val;
}
flexible.px2rem = function(d) {
    var val = parseFloat(d) / this.rem;
    if (typeof d === 'string' && d.match(/px$/)) {
        val += 'rem';
    }
    return val;
}

})(window, window['lib'] || (window['lib'] = {}));

简单版本

function resetFontSize() {
        const baseFontSize = 75
        const designWidth = 750
        const width = window.innerWidth
        const currentFontSize = (width / designWidth) * baseFontSize
        document.getElementsByTagName('html')[0].style.fontSize = currentFontSize + 'px'
      }
      window.onresize = function () {
        resetFontSize()
      }
      resetFontSize()