[css-inline-3] Clarify that hanging punctuation is not measured for initial letter box sizing. w3c#310

Author: fantasai

css-inline-3/Overview.bs

@@ -603,7 +603,9 @@ Layout Model</h4>
 		<li>
 			Size the <a>initial letter</a>’s content box.
 			The content box of a non-atomic inline initial letter
-			is the smallest rectangle required to include the entirety of its glyphs
+			is the smallest rectangle required to include the entirety of its glyphs--
+			excluding any that <a spec="css-text-3">hang</a>
+			(see 'hanging-punctuation')--
 			as well as the margin boxes of any atomic inlines it contains.
 
 		<li>