By default the Nokia Lumia 920 runs an IE10 browser which does support HTML5 geolocation.
You can check this using the link on w3schools.com. Clicking the try it button, it prompted me for my location as you would expect. The site also helpfully provides the code.
The HERE Maps API for JavaScript offers the nokia.maps.positioning.Manager
which wraps this nicely and also works in mobile - try the example here