Form behaving differently on phone than in resized browser. How do I replicate this?

I coded a simple form, tested it on my pc in different browsers and browser sizes and it worked fine. When I open the page in my phone's browser (iPhone 6, Safari) the form behaves differently.

How do I replicate this behaviour in my browser so I can debug and fix the issue? I don't know why for example the label 'verende zadelpen' is behaving this way, but I need to fix it.

example of what's happening

Is there a way to replicate exactly this, in my browser? I don't know any other way to check what classes are causing this.