[css-fonts-4] Add issue for system-ui fingerprinting

Author: fantasai

css-fonts-4/Overview.bs

@@ -131,13 +131,16 @@ used across a wide variety of platforms, it's useful to extend the set
 of predefined CSS generic font families to cover these type styles.</p>
 
 <p>Add new generic font families for:</p>
-<ul>
-<li><code>system-ui</code> font family representing the default UI font on a given platform</li>
-<li><code>emoji</code> font family for emoji characters</li>
-<li><code>math</code> font family used for rendering mathematical expressions</li>
-<li><code>fangsong</code> font family used for <em>fang song</em> typefaces in Chinese</li>
+<ul dfn-for=font-family dfn-type=value>
+<li><dfn>system-ui</dfn> font family representing the default UI font on a given platform</li>
+<li><dfn>emoji</dfn> font family for emoji characters</li>
+<li><dfn>math</dfn> font family used for rendering mathematical expressions</li>
+<li><dfn>fangsong</dfn> font family used for <em>fang song</em> typefaces in Chinese</li>
 </ul>
 
+Issue: Add ''system-ui'' fingerprinting issue to Security and Privacy Considerations section
+per <a href="https://lists.w3.org/Archives/Public/www-style/2015Aug/0051.html">discussion</a>.
+
 <h2 id="font-synthesis-small-caps">New <code>small-caps</code> value for <code>font-synthesis</code></h2>
 
 <p>Since small capitals are often synthesized for fonts lacking smalls-caps glyphs,