[css-properties-values-api] Describe substitution behavior. (w3c#863)

* [css-properties-values-api] Describe substitution behavior.

* fixup.

Author: andruud

css-properties-values-api/Overview.bs

@@ -767,6 +767,42 @@ declaration is <a spec=css-variables>invalid at computed-value time</a>.
 
 Note: This applies regardless of whether or not the fallback is being used.
 
+
+Substitution {#substitution}
+----------------------------
+
+Like unregistered custom properties, the value of a registered custom property
+can be substituted into another value with the ''var()'' function. However,
+registered custom properties substitute as their
+[[#calculation-of-computed-values|computed value]], rather than the original
+token sequence used to produce that value.
+
+Any ''var()'' function that references a registered custom property must be
+replaced with an <dfn>equivalent token sequence</dfn>, which is equal
+to the token sequence that
+would have been produced by [[cssom#serialize-a-css-value|serializing]]
+the computed value, and [[css-syntax-3#tokenization|tokenizing]] the resulting
+string.
+
+<div class='example'>
+	Suppose that '--x' is registered with ''&lt;length>'' syntax, and that '--y'
+	is an unregistered custom property.
+
+	<pre class='lang-css'>
+
+	div {
+		font-size: 10px;
+		--x: 8em;
+		--y: var(--x);
+	}
+	</pre>
+
+	Because the computed value of '--x' (when serialized)  is "80px", the computed
+	value of '--y' is: <<whitespace-token>> followed by
+	<<dimension-token>> with a value of '80' and unit 'px'.
+
+</div>
+
 Examples {#examples}
 ====================