[css-syntax] Trim whitespace from the beginning/ending of a declaration's value. Fixes #774. (#1986)

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2386,7 +2386,7 @@ Consume a declaration</h4>
 	<a>Consume the next input token</a>.
 	Create a new declaration
 	with its name set to the value of the <a>current input token</a>
-	and its value initially set to the empty list.
+	and its value initially set to the empty [=list=].
 
 	<ol>
 		<li>
@@ -2400,6 +2400,10 @@ Consume a declaration</h4>
 
 			Otherwise, <a>consume the next input token</a>.
 
+		<li>
+			While the <a>next input token</a> is a <<whitespace-token>>,
+			<a>consume the next input token</a>.
+
 		<li>
 			As long as the <a>next input token</a> is anything other than an <<EOF-token>>,
 			<a>consume a component value</a>
@@ -2412,6 +2416,10 @@ Consume a declaration</h4>
 			remove them from the declaration's value
 			and set the declaration's <var>important</var> flag to true.
 
+		<li>
+			While the last token in the declaration's value is a <<whitespace-token>>,
+			[=list/remove=] that token.
+
 		<li>
 			Return the declaration.
 	</ol>
@@ -3427,6 +3435,8 @@ Changes from the 20 February 2014 Candidate Recommendation</h3>
 
 	* If the very first rule in the sheet is a ''@charset'', dropped it during parsing.
 
+	* Trimmed whitespace from the beginning/ending of a declaration's value during parsing.
+
 	The following editorial changes were made:
 
 	* The "Consume a string token" algorithm was changed to allow calling it without specifying an explicit ending token,