CSS: fit textarea to its parent div

textarea {
    height: 100%;
    width: 100%;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

Example HTML code:

<div style="width:600px; height:400px;">
<textarea style="height:100%; width:100%; box-sizing: border-box;" readonly>
This is a textarea.
</textarea>
</div>

Run the Example:

This is a textarea.