Is it possible to prevent iPhone/iPad orientation changing in the browser?

You can detect orientation changes with onorientationchange.

Javascript:

/*window.orientation returns a value that indicates whether iPhone is in portrait mode,    landscape mode*/
window.onorientationchange = function() {
var orientation = window.orientation;

switch(orientation) {
    case 0:
         //Portrait mode
    case 90: 
         // Landscape left
    case -90:
         //Landscape right
}

It's written in iPhone doc. Here from iPhone docs.

It does not appear to be possible to prevent orientation change, but you can detect it with the code mentioned above.

If you want to prevent orientation change it appears as though you'll need to build a native app using WebKit which quashes the event.