Each element in the top layer's stack has a ::backdrop pseudo-element. This pseudo-element is a box rendered immediately below the element (and above the element below the element in the stack, if any), within the same top layer.
The :-moz-tree-row(hover) CSS pseudo-class will match an element if the mouse cursor is presently hovering over a tree row.
The backdrop-filter property provides for effects like blurring or color shifting the area behind an element, which can then be seen through that element by adjusting the element's transparency/opacity.
document.designMode controls whether the entire document is editable. Valid values are "on" and "off". According to the specification, this property is meant to default to "off". Firefox follows this standard. The earlier versions of Chrome and IE default to "inherit". Starting in Chrome 43, the default is "off" and "inherit" is no longer supported. In IE6-10, the value is capitalized. 
Accepts two optional parameters: Image([unsigned long width, unsigned long height])
The Media Capture and Streams API, often called the Media Stream API or the Stream API,  is an API related to WebRTC which supports streams of audio or video data, the methods for working with them, the constraints associated with the type of data, the success and error callbacks when using the data asynchronously, and the events that are fired during the process.
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 Web Performance API allows web pages access to certain functions for measuring the performance of web pages and web applications, including the Navigation Timing API and high-resolution time data.