[css-fonts-4] Clarify valid ranges for font properties

#1131

Author: Litherum

css-fonts-4/Overview.bs

@@ -443,7 +443,7 @@ at least as dark as its predecessor. Only values greater than 0 and less than 10
 	</tbody>
 	</table>
 
-	Negative values are not allowed and are treated as parse errors.
+	Values less than or equal to 0 are not allowed and are treated as parse errors.
 	When a face does not exist
 	for a given width, values less than 100% map to a narrower face,
 	otherwise a wider face. Conversely, values greater than or equal to 100% map to a wider