[css-text-4] Explicitly note that trimming can use halt or vhal features, and must not use hwid/halfwidth substitutions. w3c#535

Author: fantasai

css-text-4/Overview.bs

@@ -1080,9 +1080,17 @@ Fullwidth Punctuation Collapsing</h4>
 			if they are given full-width and must be set half-width, or
 		<li>
 			add space to the glyphs,
-			if they are given half-width must be set full-width.
+			if they are given half-width and must be set full-width.
 	</ul>
 
+	The UA <em>may</em> use the OpenType <code>halt</code> and <code>vhal</code> features
+	if implemented by a font
+	in order to perform the requisite trimming of a particular glyph.
+	The UA <em>must not</em> use the <code>hwid</code> feature
+	or otherwise substitute halfwidth forms
+	as switching to halfwidth glyphs can change the glyph shape
+	which is not acceptable here.
+
 	Some fonts use proportional glyphs for fullwidth punctuation characters.
 	For such proportional glyphs, the given advance width is considered
 	simultaneously full-width and half-width: no space is added or removed.