box sizing css

-webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
-moz-box-sizing: border-box;    /* Firefox, other Gecko */
box-sizing: border-box;         /* Opera/IE 8+ */

html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}

Sintaxe formal: 
content-box | (en-US) border-box