Can you build a template web app which works with a mobile devices location services?

[EllenWildig]

I'm looking to build a fairly basic web-app which gives information about features of a certain location. I have a colleague going on holiday to an area i know well so was going to log a set of 'Easter Eggs' that he might miss. However, I was hoping to publish the app so that it works in a similar way to the geocaching app. Whereby, when you get close to a location, the app recognises it is close to that point and automatically brings up information about it. Is this possible when building a simple web-app using a template in ArcGIS online? I've never done it before so really have no idea.

Thanks