https://estrella-kim.github.io/js/js/scale.fix.js

1var metas = document.getElementsByTagName('meta');
2var i;
3if (navigator.userAgent.match(/iPhone/i)) {
4  for (i=0; i<metas.length; i++) {
5    if (metas[i].name == "viewport") {
6      metas[i].content = "width=device-width, minimum-scale=1.0, maximum-scale=1.0";
7    }
8  }
9  document.addEventListener("gesturestart", gestureStart, false);
10}
11function gestureStart() {
12  for (i=0; i<metas.length; i++) {
13    if (metas[i].name == "viewport") {
14      metas[i].content = "width=device-width, minimum-scale=0.25, maximum-scale=1.6";
15    }
16  }
17}