Grammar

Comma for readability.

Author: him2312

3-frames-and-windows/01-popup-windows/article.md

@@ -13,7 +13,7 @@ window.open('https://javascript.info/')
 
 Popups exist from really ancient times. The initial idea was to show another content without closing the main window. As of now, there are other ways to do that: JavaScript is able to send requests for server, so popups are rarely used. But sometimes they are still handy.
 
-In the past evil sites abused popups a lot. A bad page could open tons of popup windows with ads. So now most browsers try to block popups and protect the user.
+In the past, evil sites abused popups a lot. A bad page could open tons of popup windows with ads. So now most browsers try to block popups and protect the user.
 
 **Most browsers block popups if they are called outside of user-triggered event handlers like `onclick`.**