[css-values] First stab at trying to fix the definition of compatible units... deal with 'em' units in example of multiplicative factor, and split out used values because they additional combine percentages/numbers/keywords/etc.

Author: fantasai

css-values/Overview.bs

@@ -808,15 +808,27 @@ Numbers with Units: <a>dimensions</a></h3>
 	resolutions (<<resolution>>),
 	and other quantities.
 
-	When serializing <a lt="computed value">computed</a> and <a>used values</a>,
+<h3 id="compat">
+Compatible Units</h3>
+
+	When serializing <a>computed values</a>,
 	<dfn export local-lt=compatible>compatible units</dfn>
 	(those related by a static multiplicative factor,
 	like the 96:1 factor between ''px'' and ''in'',
-	or the 1000:1 factor between ''ms'' and ''s'')
+	or the the computed 'font-size' factor between ''em'' and ''px'')
 	are converted into a single <dfn export local-lt=canonical>canonical unit</dfn>.
 	Each group of compatible units defines which among them is the <a>canonical unit</a>
 	that will be used for serialization.
 
+	When serializing <a href="https://www.w3.org/TR/cssom-1/#resolved-values">resolved values</a>
+	that are <a>used values</a>,
+	all value types (percentages, numbers, keywords, etc.)
+	that represent lengths are considered <a>compatible</a> with lengths.
+	Likewise any future API that returns <a>used values</a>
+	must consider any values represent distances/durations/frequencies/etc.
+	as <a>compatible</a> with the relevant class of <a>dimensions</a>,
+	and canonicalize accordingly.
+
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@@ -1608,7 +1620,7 @@ Computed Value</h4>
 		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.
+		and combining terms with identical units.
 
 		At this time, all units can be absolutized
 		to a single unit per type at computed-value time,