[css-text-3] Add examples of reasonable, unreasonable, and impossible…

… shaping across formatting changes. #698

css-text-3/Overview.bs

@@ -2125,6 +2125,41 @@ Cursive Scripts</h4>
   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.
 
+  <div class="example">
+    An example of reasonable and possible shaping across boundaries
+    is Arabic shaping:
+    in many systems this is performed by the font engine,
+    allowing the font to provide variant glyphs
+    with potentially very sophisticated contextual shaping.
+    It's not generally possible to rely on this system across a font change
+    unless the font engine has an API to provide context,
+    but it is straightforward and therefore quite reasonable
+    for an engine to work around this limitation by, for example,
+    using the zero-width-joiner (U+200D) or zero-width-non-joiner (U+200C)
+    as appropriate to solicit the correct choice of
+    initial/medial/final/isolated glyph.
+
+    An example of possible but not reasonable shaping across boundaries
+    is handling a font that is sensitive to 20 characters of context
+    on either side to choose its glyphs:
+    passing all the text before <em>and after</em> the string in question,
+    even through multiple inline boundaries with formatting changes,
+    is complicated.
+    The UA <em>could</em> handle such cases,
+    but is not required to,
+    as they are not typical or fundamentally required
+    by any modern writing system.
+
+    An example of impossible shaping accross boundaries
+    is a change in font weight partway through the word “and”
+    in a font where a ligature would replace
+    all three letters of the word “and”
+    with an ampersand glyph (“&amp;”).
+<!--
+    It's simply not possible for the UA
+    to create the effect of a partway-bold single glyph.
+-->
+  </div>
 
 <h2 id="edge-effects">
   Edge Effects</h2>