[css-writing-modes] Degrade requirement full-width -> proportional translation for text-combine-upright, according to WG discussion.

Author: fantasai

css-writing-modes/Overview.bs

@@ -2622,7 +2622,8 @@ Full-width Characters</h5>
 
   <p>In order to preserve typographic color when compressing the text to 1em,
     when the combined text consists of more than one <a>typographic character unit</a>,
-    then any full-width <a>typographic character units</a> must first be converted to their non-full-width equivalents
+    then any full-width <a>typographic character units</a>
+    should first be converted to their non-full-width equivalents
     by reversing the algorithm defined for ''text-transform: full-width'' in [[!CSS3TEXT]]
     before applying other compression techniques.