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;
}

From: http://paulirish.com/2012/box-sizing-border-box-ftw/