Back in the days when Internet Explorer 6 reigned supreme, when Firefox was just getting its foothold and taking shares from IE, and Chrome was barely a Googler’s pet project, the browsers followed different box models.
Firefox (as well as the then little-known browsers Opera and Safari) followed the W3C standard box model, what is now known as “content-box”. Internet Explorer, on the other hand, followed its own box model, what we now call “border-box”. As Firefox began to rise and threatened IE’s market share, Web developers were singing the praises of the content-box style box model (which was one of the many W3C standards that IE ignored, and Firefox implemented), to the point that when Microsoft finally released Internet Explorer 7, it implemented the standard box model.