Textarea is one of the largely used element on the web. With the increase of mobile usage, it is now a general practice to make the web forms responsive. One of the problems that I face while designing responsive webforms is when I give the textarea width:100% of the container element with padding, it stretches out of the parent container. To fix this you can use the css box-sizing property like the following.

View Demo

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

With the use of border box the textarea with padding would fit the container element and it will also respect the padding and width of the container.