Improve way to set box-sizing.

sass/base/_normalize.scss

@@ -13,10 +13,16 @@
 // ==========================================================================
 
 // Use the saner border-box model for all elements.
-* {
+html {
   box-sizing: border-box;
 }
 
+*,
+*::before,
+*::after {
+  box-sizing: inherit;
+}
+
 // Normalize-scss is broken into modular pieces to make it easier to edit.
 @import 'document/document';
 @import 'sections/sections';