[css-scroll-snap] Fix up computed value. Make it animate across percentages.

Author: fantasai

css-scroll-snap/Overview.bs

@@ -443,8 +443,9 @@ Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
 	Applies to: all elements
 	Inherited: no
 	Percentages: refer to size of scroll container's <a>snap viewport</a> <em>minus</em> size of box's <a>snap area</a>
-	Computed value: as specified
-	Animatable: no
+	Computed value: an optional ''point'' keyword, plus one or a pair of alignment values
+	                each consisting of either a ''none'' or ''edges'' keyword or a percentage
+	Animatable: as percentage, if only the percentage differs
 	Media: interactive
 	</pre>
 
@@ -482,18 +483,21 @@ Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
 		<pre>img { scroll-snap-align: center; }</pre>
 	</div>
 
-	'scroll-snap-align' has four distinct families of values,
-	depending on what <a>snap positions</a> you wish the element to declare:
+	'scroll-snap-align' has four distinct syntaxes,
+	depending on what <a>snap positions</a> you wish the element to specify:
 
 	<dl>
 		<dt id=scroll-snap-align-none>none
 		<dd>
-			The element does not declare any <a>snap positions</a> at all.
+			The element does not specify any <a>snap positions</a> at all,
+			i.e. both axes compute to ''scroll-snap-align/none''.
 
 		<dt id=scroll-snap-align-single>[ block | inline ]? [ edges | <<snap-alignment>> ]
 		<dd>
 			The element declares a single <a>1D snap position</a>,
-			in either the <a>block</a> or <a>inline</a> axis.
+			in either the <a>block</a> or <a>inline</a> axis,
+			as specified,
+			and the other axis computes to ''scroll-snap-align/none''.
 
 			If neither <css>block</css> nor <css>inline</css> is specified,
 			the axis is automatically chosen:
@@ -558,6 +562,7 @@ Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
 			where the percentage represents a position between
 			''scroll-snap-align/start'' alignment and ''scroll-snap-align/end'' alignment.
 			''0%'' matches the given alignment keyword (''scroll-snap-align/start'' or ''scroll-snap-align/end'').
+			Percentages from the ''end'' compute to start-based percentages (as 100% &minus; <<percentage>>).
 
 		<dt><dfn>edges</dfn>
 		<dd>