[web-animations-1][editorial] Various clarifications and dfn improvements to timelines

* Shift high-level description of timing model
  from “Time Values” section to “Timing Model” intro.
* Dfn “timing node” since it's used without definition,
  and cross-link it from relevant places.
* Rephrase “time value” dfn to work better with future time value types,
  stealing wording from the definition of “timeline”.
* Clarify the purpose of a timeline.
* Improve dfn of “unresolved” time values.
* Add proper dfn for “active timeline”.
* Clarify why document timeline offsets are called “origin time”.
* Improve cross-linking throughout.

Author: fantasai

web-animations-1/Overview.bs

@@ -530,6 +530,13 @@ Timing model {#timing-model}
   This section describes and defines the behavior
   of the <dfn>Web Animations [=timing model=]</dfn>.
 
+  Timing in the [=Web Animations model=] is represented through
+  a hierarchy of relationships between <dfn>timing nodes</dfn>,
+  in which parent nodes provide timing information to their child nodes
+  in the form of [=time values=],
+  ultimately resulting in an [=iteration progress=] and [=iteration index=]
+  supplied to the [=animations model=]
+
 Timing model characteristics {#timing-model-overview}
 ----------------------------
 
@@ -541,7 +548,8 @@ Timing model characteristics {#timing-model-overview}
 
 ### Stateless ### {#stateless}
 
-  The Web Animations [=timing model=] operates by taking an input time
+  The Web Animations [=timing model=] operates
+  by taking an input [=time value=]
   and producing an output [=iteration progress=].
   Since the output is based solely on the input time
   and is independent of previous inputs,
@@ -629,13 +637,11 @@ Timing model characteristics {#timing-model-overview}
 Time values {#time-value-section}
 -----------
 
-  Timing is based on a hierarchy of time relationships between timing nodes.
-  Parent nodes provide timing information to their child nodes
-  in the form of [=time values=].
-
-  A <dfn>time value</dfn> is a real number which nominally represents
+  A <dfn>time value</dfn> in the [=Web Animations model=]
+  is a value that serves the purpose of synchronization.
+  [=Time values=] are real numbers that nominally represent
   a number of milliseconds from some moment.
-  The connection between [=time values=] and wall-clock milliseconds
+  However, the connection between [=time values=] and wall-clock milliseconds
   can be obscured by any number of transformations applied to the value
   as it passes through the time hierarchy.
 
@@ -644,25 +650,30 @@ Time values {#time-value-section}
   in which case the connection between time values and milliseconds
   will be weakened even further.
 
-  A [=time value=] is <dfn>unresolved</dfn> if, for example,
-  a timing node is not in a state to produce a [=time value=].
+  A [=time value=] can be <dfn>unresolved</dfn>,
+  i.e. represent the absence of a meaningful [=time value=].
+  This can happen if
+  a [=timing node=] is not in a state to produce a [=time value=].
 
 Timelines {#timelines}
 ---------
 
-  A <dfn export>timeline</dfn> provides a source of [=time values=]
-  for the purpose of synchronization.
-
+  A <dfn export>timeline</dfn> is a [=timing node=]
+  that provides a source of [=time values=].
   At any given moment, a [=timeline=] has a single current [=time value=]
   known simply as the timeline's <dfn export for=timeline>current time</dfn>.
+  Changes in a [=timeline=]’s [=timeline/current time=]
+  are ultimately what drive [=animations=] in the [=Web Animations model=].
 
   A [=timeline=] might not always be able to return a meaningful [=time value=],
   but only an [=unresolved=] time value.
-  For example, it might be defined relative to a moment that has yet to occur,
+  For example, its [=timeline/current time=] might be defined relative to
+  a moment that has yet to occur,
   such as the firing of a document's load event.
   A [=timeline=] is considered to be
-  <dfn export lt="inactive timeline | active timeline">inactive</dfn>
-  when its [=time value=] is [=unresolved=].
+  <dfn export lt="inactive timeline">inactive</dfn>
+  when its [=time value=] is [=unresolved=],
+  and <dfn export lt="active timeline">active</dfn> otherwise.
 
   A [=timeline=] is
   <dfn export lt="monotonically increasing timeline" local-lt="monotonically increasing">monotonically increasing</dfn>
@@ -757,7 +768,10 @@ Timelines {#timelines}
   and whose [=timeline/current time=] is calculated
   as a fixed offset from the |now| timestamp provided
   each time the [=update animations and send events=] procedure is run.
-  This fixed offset is referred to as
+  This fixed offset is equal to
+  the [=timeline/current time=] of the [=default document timeline=]
+  when this timeline’s [=timeline/current time=] was zero,
+  and is thus referred to as
   the document timeline's <dfn>origin time</dfn>.
 
   Issue(2079): There must be a better term than "origin time"&mdash;
@@ -793,7 +807,7 @@ Timelines {#timelines}
 <em>This section is non-normative</em>
 
   Since no scaling is applied to
-  the |now| timestamp values provided to [=document timelines=],
+  the |now| timestamp values provided to a [=document timeline=],
   the [=time values=] it produces will be
   proportional to wall-clock milliseconds.