[css-properties-values-api] Disallow leading '|' in syntax strings. (#862)

The current text allows syntax strings like "|<color>", which seems pretty
pointless.

Author: andruud

css-properties-values-api/Overview.bs

@@ -597,12 +597,15 @@ Parsing the syntax string {#parsing-syntax}
 			otherwise, return failure.
 
 		:   U+007C VERTICAL LINE (|)
-		:: <a>Consume a syntax component</a> from |stream|.
+		:: If |descriptor|’s [=list/size=] is greater than zero,
+			<a>consume a syntax component</a> from |stream|.
 			If failure was returned,
 			return failure;
 			otherwise,
 			[=list/append=] the returned value to |descriptor|.
 
+			If |descriptor|’s [=list/size=] is zero, return failure.
+
 		:   anything else
 		:: <a>Reconsume the current input code point</a> in |stream|.
 			<a>Consume a syntax component</a> from |stream|.