[css-backgrounds-3][editorial] Negative values are invalid for 3rd <shadow> length (#12761)

Author: cdoublev

css-backgrounds-3/Overview.bs

@@ -3342,7 +3342,7 @@ Border Image Shorthand: the 'border-image' property</h3>
 	represented by 2-4 length values, an optional color, and an optional ''box-shadow/inset'' keyword.
 	Omitted lengths are 0; omitted colors default to ''currentColor''.
 	<pre class=prod>
-	<dfn><<shadow>></dfn> = <<color>>? &amp;&amp; [<<length>>{2} <<length [0,&infin;]>>? <<length>>?] &amp;&amp; inset?</pre>
+	<dfn><<shadow>></dfn> = <<color>>? &amp;&amp; [ <<length>>{2} [ <<length [0,&infin;]>> <<length>>? ]? ] &amp;&amp; inset?</pre>
 
 	The components of each <<shadow>> are interpreted as follows:
 
@@ -4132,4 +4132,4 @@ Changes Since the 17 December 2009 Candidate Recommendation</h3>
 	color-mix-currentcolor-outline-repaint-parent.html
 	color-mix-currentcolor-outline-repaint.html
 	first-letter-space-not-selected.html
-</wpt>
+</wpt>