[css-text] Define Arabic/etc shaping across inline element boundaries, per WG resolution https://lists.w3.org/Archives/Public/www-style/2014Oct/0259.html

Author: tabatkins

css-text/Overview.bs

@@ -2224,6 +2224,24 @@ Word Spacing: the 'word-spacing' property</h3>
       unless they are prepared to accept non-interoperable results.
     </div>
 
+  <h3 id="boundary-shaping">
+  Shaping Across Element Boundaries</h3>
+
+  Text shaping <em>must</em> be broken at inline box boundaries
+  when any of the following are true
+  for any box whose boundary separates the two <a>typographic character units</a>:
+
+  1. Any of 'margin'/'border'/'padding'
+    separating the two <a>typographic character units</a> in the inline axis
+    is non-zero.
+  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>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.
+
+
 <h2 id="edge-effects">
   Edge Effects</h2>