How to make an element width: 100% minus padding?

This is why we have box-sizing in CSS.

I’ve edited your example, and now it works in Safari, Chrome, Firefox, and Opera. Check it out: http://jsfiddle.net/mathias/Bupr3/ All I added was this:

input {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}

Unfortunately older browsers such as IE7 do not support this. If you’re looking for a solution that works in old IEs, check out the other answers.

box-sizing: border-box is a quick, easy way to fix it:

This will work in all modern browsers, and IE8+.

Here's a demo: http://jsfiddle.net/thirtydot/QkmSk/301/

.content {
    width: 100%;
    box-sizing: border-box;
}

The browser prefixed versions (-webkit-box-sizing, etc.) are not needed in modern browsers.