[css-fonts-4] Not all generic font families have to map to a font

Closes w3c#4442 (comment)

Author: litherum

css-fonts-4/Overview.bs

@@ -233,7 +233,6 @@ Relationship Between Faces and Families
 <h4 id="generic-font-families">
 Generic font families</h4>
 
-    Each generic font family must always map to at least one matched font face.
     A generic font family is a font family which has a standard name (as defined by CSS), but which is an alias for an existing installed font family present on the system.
     However, a single generic font family may be a composite face
     combining different typefaces based on such things as
@@ -270,6 +269,13 @@ Generic font families</h4>
             Any font that fits this typographic role
             may be used to represent the generic ''serif'' family.
 
+            ''serif'' must always map to at least one matched font face.
+
+            Note: ''serif'' must always map to at least one matched font face.
+            However, no guarantee is placed on the character coverage of that font
+            face. Therefore, the font ''serif'' is mapped to may not end up being
+            used for all content.
+
             <figure>
                 <img alt="sample serif fonts" src="images/serifexamples.png" >
                 <figcaption>Sample serif fonts</figcaption>
@@ -294,6 +300,13 @@ Generic font families</h4>
             Any font that fits this typographic role
             may be used to represent the generic ''sans-serif'' family.
 
+            ''sans-serif'' must always map to at least one matched font face.
+
+            Note: ''sans-serif'' must always map to at least one matched font face.
+            However, no guarantee is placed on the character coverage of that font
+            face. Therefore, the font ''sans-serif'' is mapped to may not end up being
+            used for all content.
+
             <figure>
                 <img alt="sample sans-serif fonts" src="images/sansserifexamples.png" >
                 <figcaption>Sample sans-serif fonts</figcaption>
@@ -329,6 +342,13 @@ Generic font families</h4>
             The sole criterion of a monospace font is that all glyphs have the same fixed width.
             This is often used to render samples of computer code.
 
+            ''monospace'' must always map to at least one matched font face.
+
+            Note: ''monospace'' must always map to at least one matched font face.
+            However, no guarantee is placed on the character coverage of that font
+            face. Therefore, the font ''monospace'' is mapped to may not end up being
+            used for all content.
+
             <figure>
                 <img alt="sample monospace fonts" src="images/monospaceexamples.png" >
                 <figcaption>Sample monospace fonts</figcaption>
@@ -399,35 +419,16 @@ Generic font families</h4>
 			and there is less variation in stroke width,
 			compared to a Song style.
 			Fang Song is often used for official Chinese Government documents.
-    </dl>
 
-<h4 id="standard-font-families">
-Standard font families</h4>
-
-    Standard font families are font families which are expected to be
-    interoperable between browsers, but which may not exist on every platform
-    or operating system. They are not required to map to a concrete font
-    face. Just like generic font families, standard font families may be a
-    composite face combining different typefaces based on such things as the
-    Unicode range of the character, the content language of the containing
-    element, user preferences, system settings, etc.
-
-    Note: These standard font families are expected to exist on different
-    platforms, but be backed by different fonts. However, some CSS authors might
-    prefer to handle this kind of platform-specific fallback themselves using
-    the 'font-family!!property' property. Therefore, user agents are encouraged
-    to also expose these standard font families by other, platform-specific,
-    font-family names, in addition to the standard names given here. Authors can
-    use these additional names to control font fallback themselves between different
-    platforms.
-
-    <dl dfn-for="font-family,<generic-family>" dfn-type=value>
         <dt id="ui-serif-def"><dfn>ui-serif</dfn>
         <dd>
             This font family is used for the serif variant of the system's
             user interface. The purpose of ''ui-serif'' is to allow web
             content to integrate with the look and feel of the native OS.
 
+            Note: ''ui-serif'' is not expected to map to any font on platforms
+            without an appropriate system font.
+
             <div class="example">
                 <figure>
                     <img alt="sample ui-serif font" src="images/ui-serif.png" >
@@ -441,6 +442,9 @@ Standard font families</h4>
             user interface. The purpose of ''ui-sans-serif'' is to allow web
             content to integrate with the look and feel of the native OS.
 
+            Note: ''ui-sans-serif'' is not expected to map to any font on platforms
+            without an appropriate system font.
+
             <div class="example">
                 <figure>
                     <img alt="sample ui-sans-serif font" src="images/ui-sans-serif.png" >
@@ -455,6 +459,9 @@ Standard font families</h4>
             allow web content to integrate with the look and feel of the
             native OS.
 
+            Note: ''ui-monospace'' is not expected to map to any font on platforms
+            without an appropriate system font.
+
             <div class="example">
                 <figure>
                     <img alt="sample ui-monospace font" src="images/ui-monospace.png" >
@@ -468,6 +475,9 @@ Standard font families</h4>
             user interface. The purpose of ''ui-rounded'' is to allow web
             content to integrate with the look and feel of the native OS.
 
+            Note: ''ui-rounded'' is not expected to map to any font on platforms
+            without an appropriate system font.
+
             <div class="example">
                 <figure>
                     <img alt="sample ui-rounded font" src="images/ui-rounded.png" >