The default viewport settings – the size and scale of the viewport – in Mobile Safari/Safari for iOS are optimised for websites with a width of 980 pixels. This means that if you use a fluid/elastic layout or media queries to make your layout adaptable, Safari on iOS doesn’t take advantage of the flexibility that offers but shows the site zoomed out as if it had a fixed width.
Another issue with Safari on iOS is how text size is handled when you change orientation from portrait to landscape on your iPhone/iPod/iPad. The default behaviour is to increase text size instead of using the extra available width to display more of the site.
As a developer you can affect both of these issues (or features if you prefer to call them that).
Posted in CSS, Usability, iOS.