If I say the width is 200px, gosh darn it, it’s gonna be a 200px wide box even if I have 20px of padding. So as you know, this is NOT how the box model has worked for the past ten years. Wikipedia has a great history of this box model. Jeff Kaufman also dove into the history

This gives you the box model you want. Applies it to all elements. Turns out many browsers already use border-box for a lot of form elements (which is why inputs and textareas look diff at width:100%;) But applying this to all elements is safe and wise.