[css-inline] Use ic unit

Author: fantasai

css-inline/Overview.bs

@@ -890,12 +890,10 @@ Initial Letter Wrapping: the 'initial-letter-wrap' property</h3>
 			The 'justify-self' property can then be used to align
 			the initial letter box within the exclusion area.
 
-			The heuristic for matching the character grid is
-			1em &times; <var>n</var> + 'letter-spacing' &times; (<var>n</var> - 1),
+			The rule for matching the character grid is
+			''1ch'' &times; <var>n</var> + 'letter-spacing' &times; (<var>n</var> - 1),
 			where <var>n</var> is the size of the initial letter
-			as specified by the 'initial-letter' property,
-			and 1em represents the typical character advance width for ideographic characters
-			(and may be replaced by a more accurate value if known).
+			as specified by the 'initial-letter' property.
 
 			<figure>
 				<img src="images/CJK-Initial.001.png" width="480"