Textarea to fill a parent container exactly, with padding

This is a case where I highly recommend changing the markup (it wont even hurt the semantics):

HTML

<div id="wrap">
  <label class="textarea-label">
    <textarea></textarea>
  </label>
</div>

CSS

.textarea-label
{
  display: block;
  /* border, padding, and other styles go here,
  don't set the height, and don't use floats or positioning */
}

.textarea-label textarea
{
  border: 0 none;
  margin: 0;
  outline: 0 none;
  padding: 0;
  width: 100%;
}

You should still be able to resize the textarea vertically without issues. If you use inline-block for the label, you should be able to resize horizontally as well.

The advantage of using the label is that clicking on the label will still focus the textarea as if you had clicked on the textarea. Additionally, using the descendant selector, you'll preserve your default textarea styles when you just need a plain-ol' textarea.

I have a solution where you can have 100% width and height with padding in the text area. I use negative margins to achieve the desired effect.

HTML:

<div class="container">
    <textarea class="text"/></textarea>
</div>

CSS:

.container{
  padding: 10px
  border: 1px solid silver
}

.container .text{
  resize: none;
  outline: none;
  width: 100%;
  padding: 10px;
  border: none;
  height: 100%;
  margin: -10px;
}

Tested this working on Google Chrome and Firefox

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.