[css-inline] Require initial-letter size to be >= 1. (Fix #341.)

Author: fantasai

css-inline/Overview.bs

@@ -426,13 +426,13 @@ Creating Initial Letters: the 'initial-letter' property</h3>
 		<dd>
 			This first argument defines the size of the initial letter
 			in terms of how many lines it occupies.
-			Negative values are not allowed.
+			Values less than one are invalid.
 
 		<dt><dfn><<integer>></dfn>
 		<dd>
 			This optional second argument argument
 			defines the number of lines the initial letter should sink.
-			Values must be greater than zero.
+			Values less than one are invalid.
 			If omitted, it duplicates the first argument,
 			floored to the nearest positive whole number.
 	</dl>