The OneSignal Unity SDK by OneSignal allows developers to implement push notification services into mobile applications and web services. Content explains how to import the plugin and add the code. Platforms in which users can integrate the service include Android, Amazon, iOS, and Windows Phone.
The Internet runs on standards. Without standards, we wouldn't have the Domain Name System (DNS) which resolves hostnames to Internet Protocol (IP) addresses, or the HyperText Transfer Protocol (HTTP) which served up this very web page. Even web service APIs, which can vary wildly depending on the back-end service, depend on standards like XML and JSON to make third-party development possible. And now, the W3C is jumping into the fray with a Working Draft for a general webapp Push API.