[css-fonts-4] font-family and src no longer required for validity, as they can be added later by script. Fix w3c#9491

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2439,7 +2439,6 @@ The <dfn id="at-font-face-rule">''@font-face''</dfn> rule</h3>
 
 	This descriptor defines the font family name
 	that will be used in all CSS font family name matching.
-	It is required for the ''@font-face'' rule to be valid.
 	It overrides the font family names
 	contained in the underlying font data.
 	If the font family name is the same
@@ -2473,7 +2472,6 @@ The <dfn id="at-font-face-rule">''@font-face''</dfn> rule</h3>
 	</pre>
 
 	This descriptor specifies the resource containing font data.
-	It is required for the ''@font-face'' rule to be valid.
 	Its value is
 	a prioritized,
 	comma-separated list