Apple has sent out email to developers titled “Get ready for Safari Push Notifications”, which included the above featured image and link to the Apple Developer portal which explained more about this notification feature.
OS X Mavericks introduces a powerful new way to keep users engaged with your website. You can now use the Apple Push Notifications Service to send notifications to your website users, right on their Mac desktop — even when Safari isn’t running. Safari Push Notifications work just like push notifications for apps. They display your website icon and notification text, which users can click to go right to your website.
This feature sounds amazing and will be available on Mavericks when it comes out during this fall. The Apple developer portal has all the resources including the WWDC session “Implementing Safari Push Notifications for Websites”.