CSS box-sizing; File under "really helpful"

When it comes to layouts, proper widths define whether or not you’ll get a headache. Especially when sizing input fields or textareas, setting the proper width is a challenge. This is due to the CSS box model, which calculates margins, paddings and borders outside of the box width.

A behavior which you can fortunately change:

box-sizing: border-box;

This changes your divs and block elements to calculate paddings and borders inside the specified width.