If you're looking to unsubscribe from a website's notifications, here's how you do it:
Open your browser and click go to Chrome Settings in the top right corner of your browser window --> Show Advanced Settings --> Content Settings --> Notifications.
Here you'll get a list of all websites from which you've enabled notifications and you can choose to deny any particular website from sending you further notifications.
Alternatively, you can also load the site in your browser and then click near the domain name and you will see options for notifications here:
You can then change your notifications settings straight from here.
Click on the settings dropdown in the top right corner of the Firefox browser then go to Preferences --> Privacy & Security --> Notifications → Settings.
You can either select a particular site to “Block” or click 'Remove Site' or click 'Remove All Sites' to disable all websites from sending you notifications. If you use Remove Site you will then be asked to subscribe to that site again the next time you visit it but you will no longer receive notifications without giving permission again.
Click on the Safari menu and then Preferences.
Then scroll down to Notifications and you will all the sites you have allowed notifications for.
Follow similar settings on mobile Chrome. Click on Settings --> Notifications then find the site you want to disable notifications for.
Follow similar settings on mobile Firefox. Go to the site you want to unsubscribe from and then click on the icon next to the domain name. You will then see a dropdown with a Edit Site Settings button.
Apple does not currently support web push notifications on iOS devices. (Last updated Feb 2018)