[css-egg] Register issue about negative speech rates

Author: frivoal

css-egg/Overview.bs

@@ -335,6 +335,7 @@ This specification extends the 'voice-rate' property, so that the ''tmbl'' unit
     New Values: [normal | x-slow | slow | medium | fast | x-fast | <<speech-rate>>] || &lt;percentage&gt; 
 </pre>
 
+Issue: Should negative values be allowed for <<speech-rate>> for reversed speech?
 
 <h2 id="rainbow">
 Double Rainbow</h2>