The exact look and feel of a notification depend on multiple different factors - the device, OS, browser, etc. In order to find out how the notification would look to someone who has your browser, OS, and device, you can simply click on 'Send me a test notification' before pushing/scheduling your campaign.
Here is a compilation of previews for the three browsers and the four OS that supports web push notification. Use this as a guide when you are drafting your campaign
Windows 7 and 8.1 Desktop
Windows 10 Desktop
Notifications in Windows 10 goes to the Action Center.
Mac Desktop
Mac Notification Center
Android Mobile
1. With a banner image and short title
2. With a banner image and a longer title
3. Without a banner image and a short title
4. Without a banner image and a longer title
1. With a banner image and short title
2. With a banner image and a longer title
3. Without a banner image and short title
4. Without a banner image a longer title
1. With a banner image and short title
2.WIth a banner image and a longer title
3. Without a banner image and short title
4. Without a banner image and a longer title
1. Default Notification
2. Sticky Notification - that requires interaction
