[css-transitions-1] Revert change to make transitions fire even when values cannot be smoothly interpolated

This reverts the following changeset:
  d129f87
  https://hg.csswg.org/drafts/rev/077f1626ecc1

Author: birtles

css-transitions-1/Overview.bs

@@ -41,7 +41,7 @@ Former Editor: Chris Marrin, Apple Inc https://www.apple.com/, cmarrin@apple.com
 Issue Tracking: Bugzilla bugs for this level https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=---&status_whiteboard=defer%20to%20level%202&status_whiteboard_type=notregexp
 Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=---
 Abstract: CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.
-Status Text: <strong>This document</strong> is expected to be relatively close to last call.  While some issues raised have yet to be addressed, new features are extremely unlikely to be considered for this level. <p>The following behaviors are at risk: <ul><li><a href="#discrete-interpolation-at-risk">Interpolation in steps of property types that cannot be interpolated</a></li></ul>
+Status Text: <strong>This document</strong> is expected to be relatively close to last call.  While some issues raised have yet to be addressed, new features are extremely unlikely to be considered for this level.
 Ignored Vars: x1, x2, y1, y2
 Link Defaults: css-transforms (property) transform
 </pre>
@@ -1257,16 +1257,6 @@ partial interface GlobalEventHandlers {
 <span id="animation-of-property-types-">Animation of property types</span> {#animatable-types}
 ==============================================================================================
 
-      <p>
-        Some property types can be interpolated,
-        which means they can animate smoothly from one value to another.
-        Other property types cannot, and thus animate only in a single
-        step from one value to the other.
-      </p>
-
-Animation of interpolated property types {#interpolated-types}
---------------------------------------------------------------
-
       <p>
         When interpolating between two values,
         <var>V</var><sub>start</sub> and <var>V</var><sub>end</sub>,
@@ -1412,31 +1402,6 @@ Animation of interpolated property types {#interpolated-types}
       of shorthand properties and the ''all'' value is applied to any
       properties (in the shorthand) that can be animated.</p>
 
-Animation in steps of other property types {#step-types}
---------------------------------------------------------
-
-      <p>
-        When interpolating between two values that cannot be interpolated,
-        <var>V</var><sub>start</sub> and <var>V</var><sub>end</sub>,
-        interpolation is done using the output <var>p</var> of the timing function.
-        If <var>p</var> is less than 0.5, then the
-        result of the interpolation is
-        <var>V</var><sub>start</sub>;
-        if <var>p</var> is greater than or equal to 0.5, then the
-        result of the interpolation is
-        <var>V</var><sub>end</sub>.
-      </p>
-
-      <p class="note" id="discrete-interpolation-at-risk">
-        This is a recent change to which implementations have
-        not yet updated.  (Prior to the change CSS Transitions
-        and CSS Animations did not run on such changes.)  It's
-        possible that it won't be compatible with existing Web content.
-        If that is the case, the problem may be mitigated by restricting
-        this behavior only to CSS Animations (and not to CSS Transitions),
-        and/or restricting it to step timing functions.
-      </p>
-
 <span id="animatable-properties-">Animatable properties</span> {#animatable-properties}
 =======================================================================================
 
@@ -1449,21 +1414,15 @@ Animation in steps of other property types {#step-types}
       when the values of that property can be interpolated
       by referring to the definitions of property types
       in the <a href="#animatable-types">previous section</a>.
-      The animated value is interpolated from the from and to values when
+      Values are animatable when
       both the from and the to values of the property have the type described.
       (When a composite type such as "length, percentage, or calc" is listed,
       this means that both values must fit into that composite type.)
       When multiple types are listed in the form "either A or B",
       both values must be of the same type to be interpolable.</p>
 
-      <p>Otherwise, since the from and to values cannot be interpolated,
-      the animation is done <a href="#step-types">in a single step</a>.</p>
-
-      <p>The 'transition-*' properties defined in this specification do
-      not undergo transitions.</p>
-
       <p>For properties that exist at the time this specification was
-      developed, this specification defines how they are
+      developed, this specification defines whether and how they are
       animated.  However, future CSS specifications may define
       additional properties, additional values for existing properties,
       or additional animation behavior of existing values.  In order to
@@ -1481,15 +1440,6 @@ Animation in steps of other property types {#step-types}
       the property animates.  Such definitions override those given in
       this specification.</p>
 
-      <p class="issue" id="issue-animatable-name">
-        It no longer makes sense for this line to be called
-        "Animatable".  It should probably be renamed to "Interpolation",
-        and the "no" value renamed to "discrete" or "in steps".
-        See mailing list thread:
-        <a href="https://lists.w3.org/Archives/Public/www-style/2015May/0256.html">message 1</a>,
-        <a href="https://lists.w3.org/Archives/Public/www-style/2015May/0257.html">message 2</a>
-      </p>
-
 <span id="properties-from-css-">Properties from CSS</span> {#animatable-css}
 ----------------------------------------------------------------------------
 
@@ -1778,36 +1728,37 @@ CSS properties to have particular values.
 The cascade also allows users to disable transitions entirely
 by overriding the transition properties.
 
-
-Changes since Working Draft of 19 November 2013 {#changes}
+Changes since Working Draft of 30 November 2017 {#changes}
 ==========================================================
 
-<p>The following are the substantive changes made since the
-<a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/">Working Draft
-dated 19 November 2013</a>:</p>
-
-<ul>
-  <li>Values that cannot be interpolated are transitioned when the timing function crosses its midpoint, instead of not running transitions and changing immediately.</li>
-  <li>Canceling and interrupting of running transitions is defined much more precisely.  This includes the after-change style no longer including styles from CSS Transitions.</li>
-  <li>Completion of transitions is defined somewhat more precisely.</li>
-  <li>The transitionend event is no longer cancelable.  This is since it has no default action, so canceling it would have no meaning.  It also matches the animation events.</li>
-  <li>The {{transitionrun}}, {{transitionstart}}, and {{transitioncancel}}
-  events have been added.</li>
-  <li>The interpolation of ''shadow/inset'' values on shadow lists is no longer backwards.</li>
-  <li>A [[#conformance]] section, [[#security]] section, [[#privacy]] section, [[#accessibility]] section, and [[#idl-index]] have been added</li>
-  <li>The identifiers accepted by 'transition-property' are defined in terms of <<custom-ident>>.</li>
-  <li>Define a little bit more about when changes to computed values happen, by saying at least that implementations must not update the effects of computed values without actually updating computed values.</li>
-</ul>
-
-<p>For more details on these changes, see the version control <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log since 2015 January 26</a> and the <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>.</p>
-
-<p>For changes in earlier working drafts:</p>
-
-<ol>
-  <li>see the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/#changes">changes section in the 19 November 2013 Working Draft</a>
-  <li>see the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20130212/ChangeLog">the ChangeLog</a> for changes in previous working drafts
-  <li>For more details on these changes, see the version control change logs, which are split in three parts because of file renaming: <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log since 2015 January 26</a>, <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>, <a href="https://hg.csswg.org/drafts/log/tip/css3-transitions/Overview.src.html">change log before 2013 March 28</a>.
-</ol>
+The following are the substantive changes made since the
+<a href="https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/">Working Draft
+dated 30 November 2017</a>:
+
+*   Reverted the change that would cause some values
+    that cannot be smoothly interpolated
+    to trigger transitions using <a>discrete</a> animation
+    since it was not implemented by any user agent
+    and likely to cause compatibility problems.
+
+For more details on these changes, see the version control
+<a href="https://github.com/w3c/csswg-drafts/commits/master/css-transitions-1/Overview.bs">change log</a>.
+
+For changes in earlier working drafts:
+
+1.   See the
+     <a href="https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/#changes">changes
+     section in the 30 November 2017 Working Draft</a>.
+1.   See the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/#changes">changes
+     section in the 19 November 2013 Working Draft</a>.
+1.   See the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20130212/ChangeLog">the
+     ChangeLog</a> for changes in previous working drafts.
+1.   For more details on these changes, see the version control change logs,
+     which are split in many parts because of file renaming:
+     *   <a href="https://github.com/w3c/csswg-drafts/commits/master/css-transitions-1/Overview.bs">change log since 2017 October 12</a>,
+     *   <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log from 2015 January 26 to 2017 October 12</a>,
+     *   <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>,
+     *   <a href="https://hg.csswg.org/drafts/log/tip/css3-transitions/Overview.src.html">change log before 2013 March 28</a>.
 
 Acknowledgments {#acknowledgments}
 ==================================