Please refer to this article which implements the CSS3 box-sizing property
http://css-tricks.com/box-sizing/
A quick solution for this is to set
textarea {
width: 100%;
-webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
-moz-box-sizing: border-box; /* Firefox, other Gecko */
box-sizing: border-box; /* Opera/IE 8+ */
}
And here is a working example which implements the box-sizing property
http://css-tricks.com/examples/BoxSizing/
Using widths like 99% or somewhat lesser to adjust within the container div, has already been discouraged.