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

<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>

 

You need to define width of the div containing the textarea and when you declare textarea, you can then set .main > textarea to have width: inherit.

Note: .main > textarea means a <textarea> inside of an element with class="main".

Here is the working solution

The HTML:

<div class="wrapper">
  <div class="left">left</div>
  <div class="main">
    <textarea name="" cols="" rows=""></textarea>
  </div>
</div>

The CSS:

.wrapper {
  display: table;
  width: 100%;
}

.left {
  width: 20%;
  background: #cccccc;
  display: table-cell;
}

.main {
  width: 80%;
  background: gray;
  display: inline;
}

.main > textarea {
  width: inherit;
}

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;
}