It’s possible to prevent webpage scaling in safari on iOS 10, but it’s going to involve more work on your part. I guess the argument is that a degree of difficulty should stop cargo-cult devs from dropping “user-scalable=no” into every viewport tag and making things needlessly difficult for vision-impaired users.
Still, I would like to see Apple change their implementation so that there is a simple (meta-tag) way to disable double-tap-to-zoom. Most of the difficulties relate to that interaction.
You can stop pinch-to-zoom with something like this:
document.addEventListener('touchmove', function (event) {
if (event.scale !== 1) { event.preventDefault(); }
}, false);
Note that if any deeper targets call stopPropagation on the event, the event will not reach the document and the scaling behavior will not be prevented by this listener.
Disabling double-tap-to-zoom is similar. You disable any tap on the document occurring within 300 milliseconds of the prior tap:
var lastTouchEnd = 0;
document.addEventListener('touchend', function (event) {
var now = (new Date()).getTime();
if (now - lastTouchEnd <= 300) {
event.preventDefault();
}
lastTouchEnd = now;
}, false);
If you don’t set up your form elements right, focusing on an input will auto-zoom, and since you have mostly disabled manual zoom, it will now be almost impossible to unzoom. Make sure the input font size is >= 16px.
If you’re trying to solve this in a WKWebView in a native app, the solution given above is viable, but this is a better solution: https://stackoverflow.com/a/31943976/661418. And as mentioned in other answers, in iOS 10 beta 6, Apple has now provided a flag to honor the meta tag.
Update May 2017: I replaced the old ‘check touches length on touchstart’ method of disabling pinch-zoom with a simpler ‘check event.scale on touchmove’ approach. Should be more reliable for everyone.