[css-break] Guarantee progress.

Author: fantasai

css-break/Overview.bs

@@ -762,6 +762,10 @@ Unforced Breaks</h3>
     <span id="monolithic-breaking">
     In such cases, the UA may also fragment the contents of <i>monolithic</i> elements
     by slicing the element's graphical representation.
+    However, the UA must not break at the top of the page,
+    i.e. it must place at least some content on each fragmentainer,
+    so that each fragmentainer has a non-zero amount of content,
+    in order to guarantee progress through the content.
   </p>
 
 

css-break/Overview.html

@@ -646,7 +646,11 @@ <h3 class="heading settled" data-level="4.4" id="unforced-breaks"><span class="s
    <p> Finally, if there are no possible break points below the top of the fragmentainer,
     and not all the content fits, the UA may break anywhere
     in order to avoid losing content off the edge of the fragmentainer. <span id="monolithic-breaking"> In such cases, the UA may also fragment the contents of <a data-link-type="dfn" href="#monolithic">monolithic</a> elements
-    by slicing the element’s graphical representation. </span></p>
+    by slicing the element’s graphical representation.
+    However, the UA must not break at the top of the page,
+    i.e. it must place at least some content on each fragmentainer,
+    so that each fragmentainer has a non-zero amount of content,
+    in order to guarantee progress through the content. </span></p>
    <h3 class="heading settled" data-level="4.5" id="best-breaks"><span class="secno">4.5. </span><span class="content"> Optimizing Unforced Breaks</span><a class="self-link" href="#best-breaks"></a></h3>
    <p>While CSS3 requires that a fragmented flow must break at allowed
     break points in order to avoid overflowing the fragmentainers in its