html - How can I make a TextArea 100% width without overflowing when padding is present in CSS?

Question

I have the following CSS and HTML snippet being rendered.

textarea
{
  border:1px solid #999999;
  width:100%;
  margin:5px 0;
  padding:3px;
}

<div style="display: block;" id="rulesformitem" class="formitem">
  <label for="rules" id="ruleslabel">Rules:</label>
  <textarea cols="2" rows="10" id="rules"/>
</div>

Answer 1

Why not forget the hacks and just do it with CSS?

One I use frequently:

.boxsizingBorder {
    -webkit-box-sizing: border-box;
       -moz-box-sizing: border-box;
            box-sizing: border-box;
}

See browser support here.