A easy script to put at your HTML body end to fix scale gestures on iOS devices

iphone-gesture-scale-fix.js

!(function(document, undefined) {  // IIFE::start
    if (!navigator.userAgent.match(/iPhone/i)) return;  // only for iPhone

    var metas = document.getElementsByTagName("meta"),
        NORMAL_SCALE = 1.0,
        MIN_SCALE = 0.25,
        MAX_SCALE = 1.6;

    initialize();
//    document.addEventListener("gesturestart", onGestureStart, false);
    document.addEventListener("touchstart", onGestureStart, false);
    document.addEventListener("touchend", onGestureEnd, false);

    function changeViewportContent(content) {
        for (var i = 0; i < metas.length; i++) {
            if (metas[i].name == "viewport") {
                metas[i].content = content;
            }
        }
    }

    function setViewportScale(min, max) {
        for (var i = 0; i < metas.length; i++) {
            if (metas[i].name == "viewport") {
                metas[i].content = "width=device-width, minimum-scale=" + (min) + ", maximum-scale=" + (max);
            }
        }
    }

    function initialize() {
        changeViewportContent("width=device-width, minimum-scale=1.0, maximum-scale=1.0");
    }

    function onGestureStart(event) {
        changeViewportContent("width=device-width, minimum-scale=0.25, maximum-scale=1.6");
    }

    function onGestureEnd(event) {
        initialize();
    };
})(document);  // IIFE::end