Mobile Safari is probably the best browser solution for iOS devices. The browser offers many advanced features and some eye-candy form elements to interact with. These eye-candy form elements already have some default webkit-only styles and can make your life a bit harder if you want to style them differently. Anyway, cutting a long story short, if you don’t want input elements to have rounded corners, just use the CSS rule shown below:
input {
-webkit-appearance: none;
}