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.

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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.