[css-values-4] Fix the type-resolution of division operations. Minor surrounding markup tweaks.

Author: tabatkins

css-values-4/Overview.bs

@@ -1776,18 +1776,27 @@ Type-Checking Expressions</h5>
 	so long as the expression as a whole resolves to a singular unit
 	according to the following rules:
 
-	* At a ''+'' or ''-'' sub-expression,
+	* At a <css>+</css> or <css>-</css> sub-expression,
 		attempt to [=add two types|add the types=] of the left and right arguments.
 		If this returns failure,
 		the entire [=math function=] is invalid.
 		Otherwise, the sub-expression's [=CSSNumericValue/type=] is the returned type.
 
-	* At a ''*'' or ''/'' sub-expression,
+	* At a <css>*</css> sub-expression,
 		attempt to [=multiply two types|multiply the types=] of the left and right arguments.
 		If this returns failure,
 		the entire [=math function=] is invalid.
 		Otherwise, the sub-expression's [=CSSNumericValue/type=] is the returned type.
 
+	* At a <css>/</css> sub-expression,
+		find the [=CSSNumericValue/types=] of its left and right arguments,
+		then negate the values of every [=map/entry=] in the right argument’s [=CSSNumericValue/type=].
+		Attempt to [=multiply two types|multiply=] the left argument’s type
+		and the negated right argument’s type.
+		If this returns failure,
+		the entire [=math function=] is invalid.
+		Otherwise, the sub-expression's [=CSSNumericValue/type=] is the returned type.
+
 	* The [=CSSNumericValue/type=] of a literal
 		is the result of [=creating a type=]
 		from the literal's unit,
@@ -1809,7 +1818,7 @@ Type-Checking Expressions</h5>
 		If the result is failure,
 		the entire [=math function=] is invalid.
 
-	The [=math function=] resolves to <<length>>, <<angle>>, <<time>>, <<frequency>>, <<resolution>>, <<flex>>, or <<percentage>>
+	A [=math function=] resolves to <<length>>, <<angle>>, <<time>>, <<frequency>>, <<resolution>>, <<flex>>, or <<percentage>>
 	according to which of those productions its [=CSSNumericValue/type=] [=CSSNumericValue/matches=].
 	(These categories are mutually exclusive.)
 	If it can't [=CSSNumericValue/match=] any of these,