[css-counter-styles-3] Make it clearer that 'numbers' uses the numeric value. Specify some obvious conditions for when words/spell-out should fall back.

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -1134,20 +1134,22 @@ Speech Synthesis: the '@counter-style/speak-as' descriptor</h3>
 
 		<dt><dfn>numbers</dfn>
 		<dd>
-			The counter's value is spoken as a number in the <a>content language</a>.
+			The counter's numeric value is spoken as a number in the <a>content language</a>.
 
 		<dt><dfn>words</dfn>
 		<dd>
 			<a>Generate a counter representation</a> for the value as normal,
 			then speak it as normal text in the <a>content language</a>.
-
+			If the counter representation contains images,
+			instead handle the value as for ''numbers''.
 
 		<dt><dfn>spell-out</dfn>
 		<dd>
 			<a>Generate a counter representation</a> for the value as normal,
 			then spell it out letter-by-letter in the <a>content language</a>.
-			If the UA does not know how to pronounce the symbols,
-			it may handle it as ''numbers''.
+			If the UA does not know how to pronounce the symbols
+			(or the counter representation contains images),
+			it must instead handle the value as for ''numbers''.
 
 			For example, ''lower-greek'' in English would be read out as "alpha", "beta", "gamma", etc.
 			Conversely, ''upper-latin'' in French would be read out as (in phonetic notation) /a/, /be/, /se/, etc.