[css-font-loading] Make the check() function differentiate between 'your font faces don't exist' and 'none of your fonts will render that example text'.

Author: tabatkins

css-font-loading/Overview.bs

@@ -716,13 +716,18 @@ Events</h3>
 			and add them to <var>matched font faces</var>.
 			The use of the {{FontFace/unicodeRange}} attribute means that this may be more than just a single font face.
 
+		<li>
+			If <var>matched font faces</var> is empty,
+			set the <var>found faces</var> flag to false.
+			Otherwise, set it to true.
+
 		<li>
 			For each font face in <var>matched font faces</var>,
 			if its defined 'unicode-range' does not include the codepoint of at least one character in <var>text</var>,
 			remove it from the list.
 
 		<li>
-			Return <var>matched font faces</var>.
+			Return <var>matched font faces</var> and the <var>found faces</var> flag.
 	</ol>
 
 <!--
@@ -766,7 +771,8 @@ The <code>load()</code> method</h3>
 		<li>
 			<a>Find the matching font faces</a> from <var>font face set</var>
 			using the {{FontFaceSet/load()/font}} and {{FontFaceSet/load()/text}} arguments passed to the function,
-			and let <var>font face list</var> be the return value.
+			and let <var>font face list</var> be the return value
+			(ignoring the <var>found faces</var> flag).
 			If a syntax error was returned,
 			reject <var>promise</var> with a SyntaxError exception
 			and terminate these steps.
@@ -817,18 +823,20 @@ The <code>check()</code> method</h3>
 			<a>Find the matching font faces</a> from <var>font face set</var>
 			using the {{FontFaceSet/check()/font}} and {{FontFaceSet/check()/text}} arguments passed to the function,
 			and including system fonts,
-			and let <var>font face list</var> be the return value.
+			and let <var>font face list</var> be the returned list of font faces,
+			and <var>found faces</var> be the returned <var>found faces</var> flag.
 			If a syntax error was returned,
 			throw a SyntaxError exception
 			and terminate these steps.
 
 		<li>
-			If the <var>font face list</var> contains no font faces,
-			return <code>false</code>.
+			If <var>found faces</var> is false,
+			throw an XXX error
+			and abort this algorithm.
 
 		<li>
-			If all fonts in the <var>font face list</var> have a {{FontFace/status}} attribute of "loaded",
-			or are system fonts,
+			If <var>font face list</var> is empty,
+			or all fonts in the <var>font face list</var> either have a {{FontFace/status}} attribute of "loaded" or are system fonts,
 			return <code>true</code>.
 			Otherwise, return <code>false</code>.
 	</ol>