Editorial: use inset property in the UA style sheet

Fixes #201.

Author: foolip

fullscreen.bs

@@ -676,7 +676,7 @@ properties apply to this pseudo-element either.
 
 *|*:not(:root):fullscreen {
   position:fixed !important;
-  top:0 !important; right:0 !important; bottom:0 !important; left:0 !important;
+  inset:0 !important;
   margin:0 !important;
   box-sizing:border-box !important;
   min-width:0 !important;
@@ -698,7 +698,7 @@ iframe:fullscreen {
 
 ::backdrop {
   position:fixed;
-  top:0; right:0; bottom:0; left:0;
+  inset:0;
 }
 
 *|*:not(:root):fullscreen::backdrop {