https://adampbeardsley.github.io/assets/js/scale.fix.js

1(function(document) {
2    var metas = document.getElementsByTagName('meta'),
3        changeViewportContent = function(content) {
4            for (var i = 0; i < metas.length; i++) {
5                if (metas[i].name == "viewport") {
6                    metas[i].content = content;
7                }
8            }
9        },
10        initialize = function() {
11            changeViewportContent("width=device-width, minimum-scale=1.0, maximum-scale=1.0");
12        },
13        gestureStart = function() {
14            changeViewportContent("width=device-width, minimum-scale=0.25, maximum-scale=1.6");
15        },
16        gestureEnd = function() {
17            initialize();
18        };
19
20
21    if (navigator.userAgent.match(/iPhone/i)) {
22        initialize();
23
24        document.addEventListener("touchstart", gestureStart, false);
25        document.addEventListener("touchend", gestureEnd, false);
26    }
27})(document);