[css-text-3] Require line breaks anywhere if lexical resource for breaking is missing. #4284

Author: fantasai

css-text-3/Overview.bs

@@ -2360,6 +2360,20 @@ Line Breaking Details</h3>
       the break occurs immediately before/after the box (at its margin edge)
       rather than breaking the box between its content edge and the content.
     <li>Line breaking in/around Ruby is defined in <a href="https://www.w3.org/TR/css-ruby-1/#line-breaks">CSS Ruby</a> [[!CSS-RUBY-1]].
+    <li>In order to avoid unexpected overflow,
+      if the User Agent is unable to perform the requisite lexical or orthographic analysis
+      for line breaking any [=content language=] that requires it--
+      for example due to lacking a dictionary for certain languages--
+      it must assume a [=soft wrap opportunity=]
+      between pairs of [=typographic letter units=] in that writing system.
+
+      Note: This provision is not triggered merely when
+      the UA fails to find a word boundary in a particular text run;
+      the text run may well be a single unbreakable word.
+      It applies for example
+      when a text run is composed of Khmer characters (U+1780 to U+17FF)
+      if the User Agent does not know how to determine
+      word boundaries in Khmer.
   </ul>
 
   <h3 id="word-break-property" caniuse="word-break" oldids="word-break">