[css-variables] Specify that you *do* eagerly syntax-check var() functions.

Author: tabatkins

css-variables/Overview.bs

@@ -373,7 +373,9 @@ Using Cascading Variables: the ''var()'' notation</h2>
 	For example, ''var(--foo, red, blue)'' defines a fallback of ''red, blue'';
 	that is, anything between the first comma and the end of the function is considered a fallback value.
 
-	A property value containing a ''var()'' function must be assumed to be valid at parse time.
+	If a property contains one or more ''var()'' functions,
+	and those functions are syntactically valid,
+	the entire property's grammar must be assumed to be valid at parse time.
 	It is only syntax-checked at computed-value time,
 	after ''var()'' functions have been <a title="substitute a var()">substituted</a>.
 

css-variables/Overview.html

@@ -444,7 +444,9 @@ <h2 class="heading settled heading" data-level=3 id=using-variables><span class=
 	For example, <span class=css data-link-type=maybe title="var(--foo, red, blue)">var(--foo, red, blue)</span> defines a fallback of <span class=css data-link-type=maybe title="red, blue">red, blue</span>;
 	that is, anything between the first comma and the end of the function is considered a fallback value.
 
-<p>	A property value containing a <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> function must be assumed to be valid at parse time.
+<p>	If a property contains one or more <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions,
+	and those functions are syntactically valid,
+	the entire property’s grammar must be assumed to be valid at parse time.
 	It is only syntax-checked at computed-value time,
 	after <a class=css data-link-type=maybe href=#funcdef-var title=var()>var()</a> functions have been <a data-link-type=dfn href=#substitute-a-var title="substitute a var()">substituted</a>.