How to expand textarea width to 100% of parent (or how to expand any HTML element to 100% of parent width)?

How to expand textarea width to 100% of parent?

I try width 100% but it is not works it expands to 100% of page what crash layout.

Here the question in visual way.

Please provide some hints.

<div>
  <div style="width: 20%; float: left;">
    <p>Some Contentsssssssssss</p>
  </div>
  <div style="float: left; width: 80%;">
    <textarea style="width: 100%; max-width: 100%;"></textarea>
  </div>
  <div style="clear: both;"></div>
</div>

 

The box model is something every web-developer should know about. working with percents for sizes and pixels for padding/margin just doesn't work. There always is a resolution at which it doesn't look good (e.g. giving a width of 90% and a padding/margin of 10px in a div with a width of under 100px).

Check this out (using micro.pravi's code): http://jsbin.com/umeduh/2

<div id="container">
    <div class="left">
        <div class="content">
            left
        </div>
    </div>
    <div class="right">
        <div class="content">
            right
            <textarea>Check me out!</textarea>
        </div>
    </div>
</div>

The <div class="content"> are there so you can use padding and margin without screwing up the floats.

this is the most important part of the CSS:

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