border-box was the original behaviour in an early draft of the specification, but it was changed to content-box before final publication. Browsers that jumped t

border-box was the original behaviour in an early draft of the specification, but it was changed to content-box before final publication. Browsers that jumped the gun and released with draft behaviour got it wrong. And it wasn’t default behaviour so much as the only behaviour – the box-sizing property that allowed web developers to control this came a long time afterwards.