[web-animations-1][web-animations-2] Simplify auto fill mode calculation in level 1

Fixes #2046.

Author: birtles

web-animations-1/Overview.bs

@@ -5427,20 +5427,11 @@ The {{AnimationEffect}} interface {#the-animationeffect-interface}
 
       When performing timing calculations
       the special string value <code>auto</code> is expanded
-      to one of the [=fill modes=] recognized by the timing model
-      as follows:
+      to [=fill mode/none=].
 
-      <dl class=switch>
-
-      : If the [=animation effect=] to which the fill mode is being applied
-          is a [=keyframe effect=],
-      ::
-          Use [=fill mode/none=] as the [=fill mode=].
-
-      : Otherwise,
-      ::  Use [=fill mode/both=] as the [=fill mode=].
-
-      </dl>
+      Note: The <code>auto</code> value exists as a forwards-compatibility
+      measure since future types of [=animation effects=] are expected to
+      produce a different computed [=fill mode=].
 
       Advisement: As described in [[#fill-behavior]],
       authors are discouraged from using indefinitely filling animations.

web-animations-2/Overview.bs

@@ -112,6 +112,8 @@ urlPrefix: https://drafts.csswg.org/web-animations-1/; type: dfn; spec: web-anim
     text: end delay
     text: end time
     text: fill mode
+    text: none; for: fill mode; url: fill-mode-none
+    text: both; for: fill mode; url: fill-mode-both
     text: in effect
     text: iteration count
     text: iteration duration
@@ -140,6 +142,9 @@ urlPrefix: https://drafts.csswg.org/web-animations-1/; type: dfn; spec: web-anim
     text: unresolved
     text: update an animation's finished state
     text: default document timeline
+urlPrefix: https://drafts.csswg.org/web-animations-1/; type: dict-member; spec: web-animations-1
+    text: fill; for: EffectTiming; url: dom-effecttiming-fill
+    text: fill; for: OptionalEffectTiming; url: dom-optionaleffecttiming-fill
 urlPrefix: https://webidl.spec.whatwg.org/; type: dfn; spec: webidl
     text: resolve a promise; url: resolve
 urlPrefix: http://www.ecma-international.org/ecma-262/6.0/#sec-; type: dfn; spec: ecma-262
@@ -2872,6 +2877,32 @@ where the unit is a valid time unit or percent.
     > <a>sequence effect</a>. The |specified end delay| is converted to an
     > <a>end delay</a> following the [=normalize specified timing=] procedure.
 
+:   The {{EffectTiming/fill}} and {{OptionalEffectTiming/fill}} members
+::  Replace:
+
+    > When performing timing calculations
+    > the special string value <code>auto</code> is expanded
+    > to [=fill mode/none=].
+
+    with:
+
+    > When performing timing calculations
+    > the special string value <code>auto</code> is expanded
+    > to one of the [=fill modes=] recognized by the timing model
+    > as follows:
+    >
+    > <dl class=switch>
+    >
+    > : If the [=animation effect=] to which the fill mode is being applied
+    >     is a [=keyframe effect=],
+    > ::
+    >     Use [=fill mode/none=] as the [=fill mode=].
+    >
+    > : Otherwise,
+    > ::  Use [=fill mode/both=] as the [=fill mode=].
+    >
+    > </dl>
+
 :   <dfn dict-member for=EffectTiming>duration</dfn><dfn dict-member
     for=OptionalEffectTiming lt=duration></dfn>
 ::  Update the description as: