[css-text-4] If font features can determine fullwidth/halfwidth glyph shapes, use those even if the font has proportional glyphs.

Author: fantasai

css-text-4/Overview.bs

@@ -1265,8 +1265,13 @@ Fullwidth Punctuation Collapsing</h4>
 	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.
+	If there is no support in the font for distinguishing
+	fullwidth vs halfwidth glyph shapes
+	(e.g. through font features),
+	then for such proportional glyphs,
+	the given advance width is considered
+	simultaneously full-width and half-width:
+	the UA must not add or remove space to these glyphs.
 
 	Note: The advance width of a standard Han character
 	can be determined either from font metrics