[css-text-3] Clarify intended rendering of split grapheme clusters while maintaining that this may not always be possible. #699

Author: fantasai

css-text-3/Overview.bs

@@ -315,8 +315,16 @@ Characters and Letters</h3>
   for how to determine the Unicode properties of a <a>typographic character unit</a>.
 
   <p>The rendering characteristics of a <a>typographic character unit</a> divided
-  by an element boundary is undefined:
-  it may be rendered as belonging to either side of the boundary,
+  by an element boundary is undefined.
+  Ideally each component should be rendered
+  according to the formatting requirements of its respective element’s properties
+  while maintaining correct shaping and positioning
+  of the <a>typographic character unit</a> as a whole.
+  However, depending on the nature of the formatting differences between its parts
+  and the capabilities of the font technology in use,
+  this is not always possible.
+  Therefore such a <a>typographic character unit</a>
+  may be rendered as belonging to either side of the boundary,
   or as some approximation of belonging to both.
   Authors are forewarned that dividing <a>grapheme clusters</a>
   by element boundaries may give inconsistent or undesired results.