[css-text] Handle consecutive line breaks in transformation rules. Fixes #836

Author: fantasai

css-text/Overview.bs

@@ -650,9 +650,13 @@ Languages and Typesetting</h4>
       <a>segment breaks</a> are not <a>collapsible</a>
       and are instead transformed into a preserved line feed (U+000A).
 
-      <p>For other values of 'white-space', <a>segment breaks</a> are <a>collapsible</a>,
-        and are either transformed into a space (U+0020) or removed
-        depending on the context before and after the break:</p>
+      <p>For other values of 'white-space', <a>segment breaks</a> are <a>collapsible</a>.
+        As with spaces,
+        any collapsible <a>segment break</a> immediately following another collapsible <a>segment break</a>
+        is removed.
+        Then the remaining <a>segment breaks</a> are
+        either transformed into a space (U+0020) or removed
+        depending on the context before and after the break:
 
       <ul>
         <li>If the character immediately before or immediately after the segment