[css-transitions-1] Make discrete properties transition at 50%; exclu…

…de them from 'all'.

See #4441

css-transitions-2/Overview.bs

@@ -72,12 +72,33 @@ spec:web-animations-1; type:dfn;
 
 ## The 'transition-property' Property ## {#transition-name-property}
 
+ISSUE: Insert text from [[css-transitions-1#transition-property-property]]
+and backport the following paragraph.
+
 Although the 'transition-property' may specify shorthand properties and the
 ''all'' keyword, individual transitions are generated for each longhand
 sub-property that is animatable. The <dfn>expanded transition property
 name</dfn> of a transition is the name of the longhand sub-property for which
 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=].
+
 ## The 'transition-duration' Property ## {#transition-duration-property}
 
 The 'transition-duration' property specifies the [=iteration duration=]