I use this bookmarklet (taken from Stack Exchange [0]) on webpages that disable pinch-to-zoom to re-enable it on iOS devices. Some pages crash Safari after using it and zooming for unknown reasons.

    javascript:document.querySelector('meta[name=viewport]').setAttribute('content','width=200,initial-scale=1.0,maximum-scale=10.0,user-scalable=1');

[0]: http://apple.stackexchange.com/questions/28391/how-can-i-for...