[css-values-4] Define textual and numeric data types more precisely.

Author: fantasai

css-values-4/Overview.bs

@@ -437,11 +437,28 @@ Combining Values: Interpolation, Addition, and Accumulation</h2>
 <h2 id="textual-values">
 Textual Data Types</h2>
 
-	<dfn export lt="CSS identifier | CSS ident | identifier | ident">Identifiers</dfn>,
+	Textual data types are used to represent identifiers or text.
+	Aside from the casing of <a href="#keywords">pre-defined keywords</a>
+	or as explicitly defined for a given property,
+	no normalization is performed,
+	not even Unicode normalization:
+	the <a lt="specified value">specified</a> and <a>computed value</a> of a property
+	are exactly the provided Unicode values after parsing
+	(which includes character set conversion and [[css-syntax-3#escaping|escaping]]).
+	[[!UNICODE]] [[!CSS3SYN]]
+
+	<dfn export lt="CSS identifier | CSS ident | identifier | ident">CSS identifiers</dfn>,
 	generically denoted by <dfn>&lt;ident></dfn>,
 	consist of a sequence of characters conforming to the <<ident-token>> grammar. [[!CSS3SYN]]
 	Identifiers cannot be quoted;
 	otherwise they would be interpreted as strings.
+	CSS properties accept two classes of <a>identifiers</a>:
+	[[#keywords|pre-defined keywords]]
+	and [[#custom-idents|author-defined identifiers]].
+
+	Note: The <<ident>> production is not meant for property value definitions--
+	<<custom-ident>> should be used instead.
+	It is provided as a convenience for defining other syntactic constructs.
 
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@@ -457,7 +474,7 @@ Textual Data Types</h2>
 Pre-defined Keywords</h3>
 
 	In the value definition fields,
-	keywords with a pre-defined meaning appear literally.
+	<dfn lt="keyword" export>keywords</dfn> with a pre-defined meaning appear literally.
 	Keywords are <a>CSS identifiers</a>
 	and are interpreted <a lt="ASCII case-insensitive">ASCII case-insensitively</a>
 	(i.e., [a-z] and \[A-Z] are equivalent).
@@ -748,6 +765,15 @@ URL Modifiers</h4>
 <h2 id="numeric-types">
 Numeric Data Types</h2>
 
+	Numeric data types are used to represent
+	quantities, indexes, positions, and other such values.
+	Although many syntactic variations can exist
+	in expressing the quantity (numeric aspect) in a given numeric value,
+	the <a lt="specified value">specified</a> and <a>computed value</a>
+	do not distinguish these variations:
+	they represent the value’s abstract quantity,
+	not its syntactic representation.
+
 		Properties may restrict numeric values to some range.
 		If the value is outside the allowed range,
 		the declaration is invalid and must be <a href="https://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.