[css-fonts-4] Move consequences of installed fonts out of Taxonomy and into Font Matching Algorithm, #5421

Author: svgeesus

css-fonts-4/Overview.bs

@@ -3840,6 +3840,26 @@ a font family is chosen
 and a particular font face is selected
 containing a glyph for that character.
 
+The set of  <a href="#installed-fonts">installed fonts</a> available 
+to the Font Matching Algorithm 
+is explicitly undefined.
+
+UAs may choose to expose all installed fonts to the web, 
+regardless of how that font was installed; 
+doing so is likely to have good internationalization properties 
+for users whose primary language is not supported 
+by fonts shipping with their operating system. 
+UAs may alternatively choose 
+to not expose user-installed fonts to aid privacy on the web; 
+for the set of installed fonts a user has installed 
+is often used as a tracking vector to track users across the web. 
+UAs may choose a hybrid approach, where some user-installed fonts 
+are exposed for internationalization, 
+but others aren't. 
+UAs are expected to make informed decisions 
+on which fonts they expose to the web, 
+so as to balance between internationalization and privacy.
+
 <wpt>
 	fallback-remote-to-data-url.html
 	fallback-url-to-local.html
@@ -7770,8 +7790,6 @@ Installed Fonts</h3>
 
 A font may be installed globally on a device. Such fonts are generally accessible in any application, even applications which have no concept of CSS. The file or files backing the font object exist on the user's device, not as a remote resource.
 
-The set of <a href="#installed-fonts">installed fonts</a> available in the <a href="#font-matching-algorithm">Font Matching Algorithm</a> is explicitly undefined.
-
 Installed Fonts must not be Web Fonts, and Web Fonts must not be Installed Fonts.
 
 <h3 id="web-fonts">
@@ -7798,7 +7816,9 @@ Such fonts are User-Installed Fonts and also are Installed Fonts.
 Web content authors should not count on the presence of user-installed fonts, 
 because there is no guarantee any user will have 
 performed the action to install a specific font. 
-User agents may choose to ignore User-Installed Fonts for the purpose of the <a href="#font-matching-algorithm">Font Matching Algorithm</a>.
+
+Please see the <a href="#font-matching-algorithm">Font Matching Algorithm</a>
+description of how user-installed fonts may interact with the font matching algorithm.
 
 Any Installed Font which is not a User-Installed font is a Preinstalled Font. It is likely that all users of a particular version of a particular Operating System will have the same set of Preinstalled Fonts installed. As such, Web content authors targeting these Operating Systems may wish to use these fonts' family names inside 'font-family!!property' properties.