Textarea to fill a parent container exactly, with padding

Use width: 100%; height: 100%; to make the <textarea> fill up the wrapping <div>. Unfortunately, you won’t be able to put on margins/padding, as they get ADDED to the 100%, so the right/bottom edges will overflow.

Edit: To use width: 100%; along with padding, you can change the box sizing model:

    width: 100%;
    height: 100%; 
    box-sizing: border-box;

With this change, 100% now refers to the distance between the outsides of the borders, instead of the distance between the outside of the content.