[css-fonts-4] Tighten first available font, fix #4796

Author: svgeesus

css-fonts-4/Overview.bs

@@ -4062,11 +4062,17 @@ given an identical set of available fonts and rendering tech.
 The <dfn export>first available font</dfn>,
 used for example in the definition of <a>font-relative lengths</a> such as ''ex''
 or in the definition of the 'line-height' property,
-is defined to be the first font
-whose 'unicode-range!!descriptor' includes the U+0020 (space) character,
+is defined to be the first font 
+for which the character U+0020 (space) 
+is not excluded by a 'unicode-range!!descriptor',
 given the font families in the 'font-family!!property' list
 (or a user agent's default font if none are available).
 
+Installed fonts referenced directly by family name, 
+rather than via @font-face rules, 
+are considered to have a 'unicode-range!!descriptor' 
+that covers the entire Unicode code space.
+
 <wpt>
 	first-available-font-001.html
 	first-available-font-002.html