Clarify event loop integration and prioritization (WICG#92)

Author: shaseley

spec/patches.md

@@ -55,6 +55,26 @@ Modify step 2.1 to read:
       [=implementation-defined=] manner.
     * |schedulerQueue|'s [=scheduler task queue/tasks=], if |schedulerQueue| is not null.
 
+Note: the HTML specification enables per-[=task source=] prioritization by making the selection of
+the next [=task=]'s [=task queue=] in the event loop processing steps [=implementation-defined=].
+Similarly, this specification makes selecting between the next {{Scheduler}} task and the next task
+from an [=event loop=]'s [=task queues=] [=implementation-defined=], which provides UAs with the
+most scheduling flexibility.
+<br/><br/>
+But the intent of this specification is that the {{TaskPriority}} of {{Scheduler}} tasks would
+influence the event loop priority. Specifically, "{{TaskPriority/background}}" tasks and
+continuations are typically considered less important than most other event loop tasks, while
+"{{TaskPriority/user-blocking}}" tasks and continuations, as well as "{{TaskPriority/user-visible}}"
+continuations (but not tasks), are typically considered to be more important.
+<br/><br/>
+One strategy is to run {{Scheduler}} tasks with an [=scheduler task queue/effective priority=] of 3
+or higher with an elevated priority, e.g. lower than input, rendering, and other <em>urgent</em>
+work, but higher than most other [=task sources=]. {{Scheduler}} tasks with an [=scheduler task
+queue/effective priority=] of 0 or 1 could be run only when no other tasks in an [=event loop=]'s
+[=task queues=] are [=task/runnable=], and {{Scheduler}} tasks with an [=scheduler task
+queue/effective priority=] of 2 could be scheduled like other scheduling-related [=task sources=],
+e.g. the [=timer task source=].
+
 Issue: The |taskQueue| in this step will either be a [=set=] of [=tasks=] or a [=set=] of
 [=scheduler tasks=]. The steps that follow only [=set/remove=] an [=set/item=], so they are
 *roughly* compatible. Ideally, there would be a common task queue interface that supports a `pop()`

spec/scheduling-tasks.md

@@ -12,17 +12,27 @@ This spec formalizes three priorities to support scheduling tasks:
   };
 </pre>
 
-<dfn enum-value for=TaskPriority>user-blocking</dfn> is the highest priority, and is meant to be
-used for tasks that are blocking the user's ability to interact with the page, such as rendering the
-core experience or responding to user input.
-
-<dfn enum-value for=TaskPriority>user-visible</dfn> is the second highest priority, and is meant to
-be used for tasks that are observable to the user but not necessarily blocking user actions, such as
-updating secondary parts of the page. This is the default priority.
-
-<dfn enum-value for=TaskPriority>background</dfn> is the lowest priority, and is meant to be used
-for tasks that are not time-critical, such as background log processing or initializing certain
-third party libraries.
+<dfn enum-value for=TaskPriority>user-blocking</dfn> is the highest priority, and it is meant for
+tasks that should run as soon as possible, such that running them at a lower priority would degrade
+user experience. This could be (chunked) work that is directly in response to user input, or
+updating the in-viewport UI state, for example.
+<br/><br/>
+Note that tasks scheduled with this priority will typically have a higher event loop priority
+compared to other tasks, but they are not necessarily render-blocking. Work that needs to happen
+immediately without interruption should typically be done synchronously &mdash; but this can lead to
+poor responsiveness if the work takes too long. "{{TaskPriority/user-blocking}}" tasks, on the other
+hand, can be used to break up work and remain remain responsive to input and rendering, while
+increasing the liklihood that the work finishes as soon as possible.
+
+<dfn enum-value for=TaskPriority>user-visible</dfn> is the second highest priority, and it is meant
+for tasks that will have useful side effects that are observable to the user, but either which are
+not immediately observable or which are not essential to user experience. Tasks with this prioriy
+are less important or less urgent than "{{TaskPriority/user-blocking}}" tasks. This is the default
+priority.
+
+<dfn enum-value for=TaskPriority>background</dfn> is the lowest priority, and it is meant to be used
+for tasks that are not time-critical, such as background log or metrics processing or initializing
+certain third party libraries.
 
 Continuation priorities mirror task priorities, with an additional option to inherit the current
 priority: