[css-fonts-3] Fix #1736

Author: svgeesus

css-fonts-3/Fonts.src.html

@@ -485,13 +485,18 @@ <h3 id="font-family-prop">Font family: the <a href="#propdef-font-family">font-f
 
 <h4 id="generic-font-families">Generic font families</h4>
 
-<p>All five generic font families are defined to exist in all CSS
-implementations (they need not necessarily map to five distinct actual
-fonts). User agents should provide reasonable default choices for the
-generic font families, which express the characteristics of each family
-as well as possible within the limits allowed by the underlying
-technology. User agents are encouraged to allow users to select
-alternative choices for the generic fonts.</p>
+<p>All five generic font families must always result in at least
+  one matched font face, for all CSS implementations. However, the
+  generics may be composite faces (with different typefaces based
+  on such things as the Unicode range of the character, the language of the containing
+  element, user preferences and system settings, among others). They are also
+  not guaranteed to always be different from each other.</p>
+
+<p>User agents should provide reasonable default choices for the
+  generic font families, which express the characteristics of each
+  family as well as possible, within the limits allowed by the underlying
+  technology. User agents are encouraged to allow users to select
+  alternative choices for the generic fonts.</p>
 
 <h5 class="no-num no-toc"> <span class="index-def" title="serif, definition of"><a name="serif-def"><dfn>serif</dfn></a></span></h5>