Safari, of course. You can set the the User Agent to fool any browser detection code into thinking you are using a device.
But why not simply use the iPhone Simulator or an actual device?
If you want to do a quick test without building, run a webserver on your Mac and you can just hit that webserver from the Mobile Safari in the iPhone Simulator or real device.
I use the Middleman static site generator (because I can use templates and layouts then) and it has a built-in test server, and Middleman re-builds the static content automatically when you modify a file. So, I just access localhost:4567 from the simulator or <my-machine>:4567 from device.
There are differences between Mobile Safari and the embedded webview, however, so you do still have to test with an actual built app.