CSS: Box Sizing
Chris Coyier erklärt Box Sizing.
textarea {
-webkit-box-sizing: border-box; / * Safari/Chrome, other WebKit * /
-moz-box-sizing: border-box; / * Firefox, other Gecko * /
box-sizing: border-box; / * Opera/IE 8+ * /
}
The border-box value (as opposed to the content-box default) makes the final rendered box the declared width, and any border and padding cut inside the box.