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;
   }
 }
 

site/content/docs/4.3/helpers/embed.md

@@ -10,7 +10,7 @@ toc: true
 
 Rules are directly applied to `<iframe>`, `<embed>`, `<video>`, and `<object>` elements; optionally use an explicit descendant class `.embed-responsive-item` when you want to match the styling for other attributes.
 
-**Pro-Tip!** You don't need to include `frameborder="0"` in your `<iframe>`s as we override that for you.
+**Pro-Tip!** You don't need to include `frameborder="0"` in your `<iframe>`s as we override that for you in [reboot]({{< docsref "/content/reboot" >}}).
 
 ## Example