To give a bit more specific answer. HTML5 allows you to get the geo coordinates, and it does a pretty decent job. Overall the browser support for geolocation is pretty good, all major browsers except ie7 and ie8 (and opera mini). IE9 does the job but is the worst performer. Checkout caniuse.com:
http://caniuse.com/#search=geol
Also you need the approval of your user to access their location, so make sure you check for this and give some decent instructions in case it's turned off. Especially for Iphone turning permissions on for Safari is a bit cumbersome.