Q:

It is possible to watch the location in the background on Mobile (iOS / Android)? 

With watch I mean using HTML5's native Geolocation watch support.  This is meant for a web app, not a native one.  Thanks for helping out :D

A:

You cannot watch the user's location from a Web App in background. 
I have checked it on Chrome (Android M), and as soon as the Web App goes to background the GPS icon disappears and the watch position process is stopped. ( Once the Web App is in the foreground again, the watch process is resumed). 
The same is happening when the device is locked.  The watch process is stopped until the user unlocks it and the Web App is in the foreground.