[mediacapture-handle] Specify event handling with algorithms (#56)

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-handle:

== Specify event handling with algorithms ==
_"The user agent fires an event"_ is under-specified, in ways that raises questions around timing.

Event handling is [tricky](https://w3ctag.github.io/design-principles/#guard-against-recursion) and requires algorithms, since JS isn't allowed to observe things progressing in background processes naturally. For external occurrences, I recommend [what WebRTC uses](https://w3c.github.io/webrtc-pc/#dfn-remotecertificates) (as a minimum):

* When _something happens_, the user agent MUST [queue a task](https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-task) that runs the following steps: 
  * Check state, and abort these steps if unnecessary
  * Update JS-observable state
  * [Fire an event](https://dom.spec.whatwg.org/#concept-event-fire) named `foo` at _target_.
  * Don't do anything after this (to avoid UAFs)

This makes it easy to abide by [the following design rule](https://w3ctag.github.io/design-principles/#state-and-subclassing): _"Where possible, use a plain [Event](https://dom.spec.whatwg.org/#event) with a specified type, and capture any state information in the [target](https://dom.spec.whatwg.org/#dom-event-target) object."_ — because state is updated on the same task the event fires on (dispatch is synchronous).

Please view or discuss this issue at https://github.com/w3c/mediacapture-handle/issues/56 using your GitHub account


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