Remove borders from iframes by default (#30620)

scss/_reboot.scss

@@ -575,6 +575,11 @@ output {
   display: inline-block;
 }
 
+// Remove border from iframe
+
+iframe {
+  border: 0;
+}
 
 // Summary
 //

scss/helpers/_embed.scss

@@ -19,7 +19,6 @@
     left: 0;
     width: 100%;
     height: 100%;
-    border: 0;
   }
 }