I was rooting around in the settings for iOS Safari the other day and stumbled upon its "Experimental Features" which act just like feature flags in any other desktop browser. This is a new feature in iOS 11 and you can find it at:
Settings > Safari > Advanced > Experimental Features
Here's what it looks like today:
Right now you can toggle on really useful things like Link Preload, CSS Spring Animations and display: contents (which Rachel Andrew wrote about a while ago). All of which could very well come in handy if you want to test your work in iOS.