The NavigatorConcurrentHardwaremixin adds to the Navigator interface features which allow Web content to determine how many logical processors the user has available, in order to let content and Web apps optimize their operations to best take advantage of the user's CPU.
A PromiseWorker is a ChromeWorker except instead of calling postMessage() to send a message, you call post(), which returns a Promise. JavaScript files imported into the worker scope and main thread scope which allows posting to the worker and receiving in the form of a promise. Like ChromeWorker objects, PromiseWorker is mostly used for js-ctypes but it is not limited to that. There is no XPCOM access, similar to ChromeWorker. This answers the question "When should I use a PromiseWorker?", and the answer is, whenever you would normally use a ChromeWorker, but want postMessage to return promises.
The W3C Push API offers some exciting new functionality for developers to use in web applications: this article provides an introduction to getting Push notifications setup and running, with a simple demo.
The endpoint read-only property of the PushSubscription interface returns a DOMString containing the subscription ID associated with the push subscription.