No browser that I know of works this way. There isn't a way to force the screen orientation. The browser itself sets the screen orientation based on the actual device orientation, and then the webapp can determine what orientation the device is in and respond accordingly.
There is a hack you can use, but I don't think you'd be very happy with the result. You can use CSS transforms to rotate the page content. It's likely not pretty, though. Do some searches, and you will find some implementations of this.
You can definitely do this with phonegap. But then your app will be native, and not a web app. You never specified whether or not this is a native app or a web app. If it's going to be native, phonegap will be of help. If not, you're out of luck - exactly what Watusiware said.