[css-transitions-2] Add transition-behavior property to specify whether transitions happen on properties with discrete animation type (w3c#9101)

This was resolved here: w3c#8857

Also revert the redefinition of the all keyword to try to solve the same problem.

Author: josepharhar

css-transitions-2/Overview.bs

@@ -84,20 +84,18 @@ the transition was generated (e.g. 'border-left-width').
 ISSUE: Update the defining instance of [=transitionable=]
 once it is ported from Level 1.
 
-In CSS Transitions Level 2,
-property values are [=transitionable=]
-<em>unless</em> they have an [=animation type=] that is [=not animatable=].
-Values with a [=discrete=] [=animation type=] <em>are</em> [=transitionable=],
-and flip at 50% progress (<var>p</var> = 0.5).
-
-However, the ''transition-property/all'' keyword
-continues to expand only to all [=transitionable=] properties
-whose [=animation type=] is not [=discrete=].
-
-Note: This is for compatibility with CSS Transitions Level 1
-(and because authors will rarely want [=discrete=] properties to transition at 50% by default).
-Note this means that the ''tranisition-property/all'' keyword here
-maps to a subset of the 'all' [=shorthand property=]’s [=longhand properties=].
+In CSS Transitions Level 2, when comparing the [=before-change style=] and
+[=after-change style=] for a given property, the property values are
+[=transitionable=] if:
+<ol>
+ <li>They have an [=animation type=] that is neither [=not animatable=] nor
+ [=discrete=], or</li>
+ <li>The 'transition-behavior' is ''allow-discrete'' and they have an
+ [=animation type=] that is [=discrete=].</li>
+</ol>
+
+Note: When values with a [=discrete=] [=animation type=] are transitioned, they
+flip at 50% progress.
 
 ## The 'transition-duration' Property ## {#transition-duration-property}
 
@@ -120,6 +118,40 @@ the reversing shortening factor.
 The 'transition-delay' property specifies the [=start delay=]
 of the transition's associated [=animation effect=].
 
+## The 'transition-behavior' Property ## {#transition-behavior-property}
+
+The 'transition-behavior' property specifies whether transitions will be
+started or not for discrete properties.
+
+<pre class="propdef">
+  Name: transition-behavior
+  Value: <<transition-behavior-value>>#
+  Initial: normal
+  Applies to: All elements
+  Inherited: no
+  Percentages: N/A
+  Computed value: as specified
+  Canonical order: per grammar
+  Animation type: not animatable
+</pre>
+
+The syntax for specifying 'transition-behavior' is as follows:
+
+<div class=prod><dfn><<transition-behavior-value>></dfn> = normal | allow-discrete</pre>
+
+When ''transition-behavior-value/normal'' is specified, transitions will not be
+started for discrete properties, only for interpolable properties. When
+''allow-discrete'' is specified, transitions will be started for discrete
+properties as well as interpolable properties.
+
+## The 'transition' Shorthand Property ## {#transition-shorthand-property}
+
+The syntax for specifying an item in the 'transition' shorthand is as follows:
+
+<div class="prod">
+  <dfn type id="single-transition">&lt;single-transition&gt;</dfn> = [ ''transition-property/none'' | <<single-transition-property>> ] || <<time>> || <<easing-function>> || <<time>> || <<transition-behavior-value>>
+</div>
+
 
 # Starting of transitions # {#starting}