[css-values] Per WG resolution, serialization always simplifies by resolving expressions and combining identical units. Move some logic to serialization rather than computation.

Author: tabatkins

css-values/Overview.bs

@@ -1492,21 +1492,31 @@ Type Checking</h4>
 Computed Value</h4>
 
 	The computed value of a ''calc()'' expression is the expression
-	with all components computed,
-	with all multiplication/division subexpressions resolved,
-	and with all addition/subtraction subexpressions resolved where both sides are the same type.
+	with all components computed.
 
 	Where percentages are not resolved at computed-value time,
 	they are not resolved in ''calc()'' expressions,
-	e.g. ''calc(100% - 100% + 1em)'' resolves to ''calc(0% + 1em)'',
-	not to ''calc(1em)''.
+	e.g. ''calc(100% - 100% + 1em)'' resolves to ''calc(1em + 0%)'',
+	not to ''1em''.
 	If there are special rules for computing percentages in a value
 	(e.g. <a href="https://www.w3.org/TR/CSS21/visudet.html#the-height-property">the <css>height</css> property</a>),
 	they apply whenever a ''calc()'' expression contains percentages.
 
-	Note: Thus, the computed value of a ''calc()'' expression can be represented
-	as either a number
-	or a tuple of a dimension and a percentage.
+	<div class=note>
+		Note: The serialization rules do not preserve the structure of the computation,
+		so implementations can simplify the expressions
+		further than what is required here
+		when storing the values internally;
+		in particular, all ''calc()'' expressions can be reduced
+		to a sum of a <<number>>, a <<percentage>>, and some <<dimension>>s,
+		eliminating all multiplication or division,
+		and combining identical units.
+
+		At this time, all units can be absolutized
+		to a single unit per type at computed-value time,
+		so at that point the ''calc()'' expression can be reduced
+		to just a <<number>>, a <<percentage>>, and a single absolute <<dimension>> of the appropriate type.
+	</div>
 
 	If this resolution process results in only a single value
 	(one <<number>>, one <<dimension>>, or one <<percentage>>),
@@ -1583,23 +1593,23 @@ Serialization</h4>
 
 		* Replacing nested ''calc()'' values with parentheses containing their contents
 		* Resolving all multiplications and divisions
-		* Combining compatible units
+		* Combining identical units
 
-		(Which units are compatible can vary based on what stage of the value computation pipeline it's in.
-		For example, as a specified value, ''px'' and ''em'' are incompatible;
-		as a computed value, they're compatible.)
+		Note: The value-computation process can transform disparate units into identical ones.
+		For example, ''em'' and ''px'' are obviously different at specified-value time,
+		but at computed-value time they're both absolutized to ''px''.
 
-		The result must be a summation of unique incompatible units.
+		The result must be a summation of unique units.
 		(Terms with a value of zero <strong>must</strong> be preserved in this summation.)
 
 	2. Serialize as a ''calc()'' containing the summation,
 		with the units ordered <a>ASCII case-insensitive</a> alphabetically,
-		numbers coming before all units,
-		and percentages coming after all units.
+		the number (if present) coming before all units,
+		and the percentage (if present) coming after all units.
 
 	<div class="example">
-		For example, ''calc(20px + 30px)'' would first simplify to a single ''px'' value,
-		then serialize as just ''50px''.
+		For example, ''calc(20px + 30px)'' would serialize as ''calc(50px)'' as a specified value,
+		or as ''50px'' as a computed value.
 
 		A value like ''calc(20px + 0%)'' would serialize as ''calc(20px + 0%)'',
 		maintaining both terms in the serialized value.