[css-text-3] Clarify that formatting changes (like text decoration) that do not affect glyph don't break shaping. #698

Author: fantasai

css-text-3/Overview.bs

@@ -2118,7 +2118,10 @@ Cursive Scripts</h4>
   2. 'vertical-align' is not ''vertical-align/baseline''.
   3. The boundary is a <a lt="bidi-isolates">bidi isolation boundary</a>.
 
-  Text shaping <em>must not</em> be broken across inline box boundaries when there is no change in formatting.
+  Text shaping <em>must not</em> be broken across inline box boundaries
+  when there is no effective change in formatting,
+  or if the only formatting changes do not affect the glyphs
+  (as in applying <a href="https://www.w3.org/TR/css-text-decoration/">text decoration</a>).
   Text shaping <em>should not</em> be broken across inline box boundaries otherwise,
   if it is reasonable and possible for that case given the limitations of the font technology.