Remove default style for <dialog> for small screens

Having a breakpoint in the user agent style sheet is unprecedented,
unexpected, and more work to maintain and test. It has also not
been implemented in Chromium.

Fixes #624.

source

@@ -109869,15 +109869,6 @@ dialog::backdrop {
   background: rgba(0,0,0,0.1);
 }
 
-/* for small devices, modal dialogs go full-screen */
-@media screen and (max-width: 540px) {
-  dialog:modal {
-    top: 0;
-    width: auto;
-    margin: 1em;
-  }
-}
-
 slot {
   display: contents;
 }