[css-values] Specify that 0 parses as a <number> if it's ambiguous whether it's a <number> or <length>. Fixes #489.

Author: tabatkins

css-values-3/Overview.bs

@@ -848,8 +848,12 @@ Distance Units: the <<length>> type</h2>
 	Lengths refer to distance measurements
 	and are denoted by <dfn id="length-value">&lt;length></dfn> in the property definitions.
 	A length is a <a>dimension</a>.
-	However, for zero lengths the unit identifier is optional
+
+	For zero lengths the unit identifier is optional
 	(i.e. can be syntactically represented as the <<number>> ''0'').
+	However, if a ''0'' could be parsed as either a <<number>> or a <<length>> in a property
+	(such as 'line-height'),
+	it must parse as a <<number>>.
 
 	Properties may restrict the length value to some range.
 	If the value is outside the allowed range,

css-values/Overview.bs

@@ -845,8 +845,12 @@ Distance Units: the <<length>> type</h2>
 	Lengths refer to distance measurements
 	and are denoted by <dfn id="length-value">&lt;length></dfn> in the property definitions.
 	A length is a <a>dimension</a>.
-	However, for zero lengths the unit identifier is optional
+
+	For zero lengths the unit identifier is optional
 	(i.e. can be syntactically represented as the <<number>> ''0'').
+	However, if a ''0'' could be parsed as either a <<number>> or a <<length>> in a property
+	(such as 'line-height'),
+	it must parse as a <<number>>.
 
 	Properties may restrict the length value to some range.
 	If the value is outside the allowed range,