Preface
When I was testing my webpage on IOS, I find that the page would be zoomed out automatically if there is some form elements and some element gains focus. Sometimes it will be really weird.
Main
After searching I found this How do you disable viewport zooming on Mobile Safari? and I add user-scalable=0 in my meta[name=viewport]. It works.
However, user can still zoom the webpage though it won’t zoom out automatically when form element gains focused.