Assuming your CSS is already happily rendering on your various size mobile device screens, you need to define the viewport in the <head> of your template.
Example, this sets the page width to be the device's screen width and an initial zoom of 100%. Initial zoom is applied at page load, but not when the orientation is changed.
<meta name="viewport" content="width=device-width, initial-scale=1.0">
By adding a maximum-scale=1.0 parameter to the viewport you will force the iPad/iPhone to maintain the zoom level and re-layout the page on orientation change. The disadvantage of this is that it will disable zooming. However, the advantage is that you can make layout adjustments with media queries to present the content in a suitable fashion for the current orientation. You can read more about viewport here: Choosing a ViewPort
Now onto media queries. You should put media queries at the bottom of your CSS file and in the order of smallest width to largest width for screen widths. For example, taken from the Html5BoilerPlate CSS example:
@media only screen and (min-width: 480px) {
/* Style adjustments for viewports 480px and over go here */
}
@media only screen and (min-width: 768px) {
/* Style adjustments for viewports 768px and over go here */
}
So all your normal styles are above this and applied first, then if the screen is 480px or wider the next block of styles are applied, then if the screen is 768px or wider the last block of styles are applied.
By combining the fixed zoom level to 1.0 and the media-queries, you can make your site responsively resize to the screen size and orientation without javascript. Obviously you need to make sure the site is then well designed so users don't need zooming. If your site is optimized for mobile this shouldn't be a problem.
Please note: other non-safari mobile browsers may re-layout the page without setting the maximum-scale on the viewport. But this behavior is inconsistent and most developers seem to cater to apple devices even if the implementation is worse than other devices. Some other devices would maintain the zoom level and recenter the viewport when the orientation changes. But all devices are ok to fix the zoom level to 1.0.
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