[css-values-4] Explicitly undefine numeric values/range. Closes w3c#4552.

Author: tabatkins

css-values-4/Overview.bs

@@ -923,6 +923,15 @@ Numeric Data Types</h2>
 	(e.g. [[css-grid-1]] introduces ''fr'' units)
 	whose usage is more localized.
 
+	The precision and supported range of numeric values in CSS
+	is <em>explicitly undefined</em>,
+	and can vary based on the property or other context a value is used in.
+	However, within the CSS specifications,
+	infinite precision and range is assumed.
+	When a value cannot be explicitly supported due to range/precision limitations,
+	it must be converted to the closest value supported by the implementation,
+	but how the implementation defines "closest" is explicitly undefined as well.
+
 <h3 id="numeric-ranges">
 Range Restrictions and Range Definition Notation</h3>
 
@@ -4550,6 +4559,7 @@ Additions Since Level 3</h3>
 	Changes since <a href="http://www.w3.org/TR/css-values-3/">CSS Values and Units Level 3</a>:
 
 	<ul>
+		<li>Explicitly undefined numeric precision/range.
 		<li>Added rules for interpolation per value type,
 			and their clarified computed values.
 		<li>Defined the <<dashed-ident>> type.