ios viewport

<meta name="viewport" content="width=device-width, initial-scale=1">

<script>
    if (navigator.userAgent.match(/iPhone/i) || navigator.userAgent.match(/iPad/i)) {

        var viewportmeta = document.querySelector('meta[name="viewport"]');
        if (viewportmeta) {
            viewportmeta.content = 'width=device-width, minimum-scale=1.0, maximum-scale=1.0, initial-scale=1.0';
            document.body.addEventListener('gesturestart', function () {
                viewportmeta.content = 'width=device-width, minimum-scale=0.25, maximum-scale=1.6';
            }, false);
        }
    }
</script>