[css-counter-styles] Mark <image> in <symbol> as at-risk, as there ar…

…e no implementation plans yet. Related to #1991.

css-counter-styles-3/Overview.bs

@@ -17,6 +17,7 @@ Abstract: This module introduces the ''@counter-style'' rule, which allows autho
 Issue Tracking: Bugzilla https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&component=Counter%20Styles&resolution=---
 Link Defaults: css-text-3 (dfn) grapheme cluster, css-pseudo-4 (selector) ::marker, dom-ls (dfn) ASCII case-insensitive
 Deadline: 2015-05-03
+At Risk: the <<image>> value in <<symbol>>
 </pre>
 <style>
 pre.stylesheet { white-space: pre-wrap; }
@@ -1063,6 +1064,11 @@ Marker characters: the 'symbols' and 'additive-symbols' descriptors</h3>
 	The <a href="https://drafts.csswg.org/css3-images/#default-object-size">default object size</a> of an image <a>counter symbol</a>
 	is a 1em by 1em square.
 
+	Note: The <<image>> syntax in <<symbol>> is currently at-risk.
+	No implementations have plans to implement it currently,
+	and it complicates some usages of ''counter()''
+	in ways that haven't been fully handled.
+
 	Note: If using identifiers rather than strings to define the symbols,
 	be aware of the syntax of identifiers.
 	In particular, ascii non-letters like "*" are not identifiers,