textareas with width: 100%; will often appear to exceed the boundaries of it’s containing element. the appropriate fix is to use the box-sizing declaration, and set it to ‘border-box’:

	-webkit-box-sizing: border-box;
	-moz-box-sizing: border-box;
	-ms-box-sizing: border-box;
	box-sizing: border-box;

Note that each of the major vendors support a vendor-specific extension, in addition to the more-direct but as-yet largely unsupported CSS3 declaration.