LibWeb: Generate ::backdrop pseudo-elements

Author: https://github.com/Gingeh
Commit: 4b9f5c6fb8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4263
Reviewed-by: https://github.com/AtkinsSJ ✅

Base/res/ladybird/about-pages/settings.html

@@ -152,7 +152,6 @@
             }
 
             dialog::backdrop {
-                /* FIXME: This doesn't work in Ladybird. */
                 background-color: rgba(0, 0, 0, 0.5);
             }