CSS3 box-sizingΒΆ

See also

reference:

http://code.tutsplus.com/tutorials/quick-tip-did-internet-explorer-get-the-box-model-right–net-12328
https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing

example:

textarea {
width: 100%;
padding: 10px;
}

THIS MAKE THE TRICK:
.borderbox {
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;
}

<div style="height: 100px; width: 100px;">
<textarea class="borderbox"></textarea>
</div>