rem

  1.第一种：

 

document.documentElement.style.fontSize = document.documentElement.clientWidth / 7.5 + 'px';    （50px）

2.第二种：

(function (doc, win) {
    var docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
        recalc = function () {
            var clientWidth = docEl.clientWidth;
            if (!clientWidth) return;
            if(clientWidth>=750){
                docEl.style.fontSize = '100px';
            }else{
                docEl.style.fontSize = 100 * (clientWidth / 750) + 'px';
            }
        };

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

（50px）

3.第三种：

!function(e) {
        function t(a) {
            if (i[a]) return i[a].exports;
            var n = i[a] = {
                exports: {},
                id: a,
                loaded: !1
            };
            return e[a].call(n.exports, n, n.exports, t),
                n.loaded = !0,
                n.exports
        }
        var i = {};
        return t.m = e,
            t.c = i,
            t.p = "",
            t(0)
    } ([function(e, t) {
        "use strict";
        Object.defineProperty(t, "__esModule", {
            value: !0
        });
        var i = window;
        t["default"] = i.flex = function(normal, e, t) {
            var a = e || 100,
                n = t || 1,
                r = i.document,
                o = navigator.userAgent,
                d = o.match(/Android[\S\s]+AppleWebkit\/(\d{3})/i),
                l = o.match(/U3\/((\d+|\.){5,})/i),
                c = l && parseInt(l[1].split(".").join(""), 10) >= 80,
                p = navigator.appVersion.match(/(iphone|ipad|ipod)/gi),
                s = i.devicePixelRatio || 1;
            p || d && d[1] > 534 || c || (s = 1);
            var u = normal ? 1 : 1 / s,
                m = r.querySelector('meta[name="viewport"]');
            m || (m = r.createElement("meta"), m.setAttribute("name", "viewport"), r.head.appendChild(m)),
                m.setAttribute("content", "width=device-width,user-scalable=no,initial-scale=" + u + ",maximum-scale=" + u + ",minimum-scale=" + u),
                r.documentElement.style.fontSize = normal ? "50px": a / 2 * s * n + "px"
        },
            e.exports = t["default"]
    }]);
flex(false, 100, 1);

（100px）