Border Box Model. It turns out that pseudo-elements are not included in the universal selector, so if you want your pseudo-elements to have a proper box-model like everything else, you should include them in the declaration. Si algún elemento de la página da problemas hacemos que vuelva al modelo de toda la vida (content-box):
elemento-que-sea{ box-sizing: content-box; }
*, *:before, *:after {
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
}