Aim

To scale the page up for precise control while formerly forbidden.
To scale the page down for better UI (and sometimes extra options) by simulating a larger screen.

Method

Save a new bookmark and have the address modified as following:
To force enabling scaling up:

javascript:(function(){var vp = document.querySelector("meta[name=viewport]"); if(vp){vp.setAttribute("content", "width=device-width,initial-scale=1,maximum-scale=4.0,minimum-scale=0.1,user-scalable=yes");}})();

To scale down (and enabling scaling up too):

javascript:(function(){var vp = document.querySelector("meta[name=viewport]"); if(vp){vp.setAttribute("content", "width=1920px,initial-scale=1,maximum-scale=4.0,minimum-scale=0.1,user-scalable=yes");}})();

Reference

https://jbguide.me/2014/12/10/use-javascript-to-enlarge-website-on-ios/