[css-writing-modes] Correct rules for finding the page progression direction. Correct rules for writing-mode/direction propagation.

Author: fantasai

css-writing-modes/Overview.bs

@@ -2309,51 +2309,67 @@ Purely Physical Mappings</h3>
   </ul>
 -->
 
-<h2 id="special-elements">
-Initial Containing Block, Page, and Special Elements</h2>
-
-<h3 id="propagate-to-icb">
-Propagating to the Initial Containing Block</h3>
-
-  <p>The values of the 'direction' and the 'writing-mode' properties of the root element
-    are propagated to the initial containing block and sets the block flow
-    direction of the initial block formatting context.
-
-  <p>If the root element has body child elements [[!HTML5]],
-    user agents must propagate the computed values of
-    the 'writing-mode' and the 'direction' properties
-    from the first body child element
-    to the root element
-    before they are propagated to the initial containing block.
-    The propagated values are treated as if they were specified on the root element.
+<h2 id="principal-flow">
+The Principal Writing Mode</h2>
+
+  <p>The <dfn export>principal writing mode</dfn> of the document
+  is determined by the 'writing-mode' and 'direction' values
+  specified on the root element.
+  This writing mode is used, for example,
+  to determine the direction of scrolling
+  and the default <a>page progression</a> direction.
+
+  <p>As a special case for handling HTML documents,
+  if the root element has a <code>&lt;body></code> child element [[!HTML5]],
+  the <a>principal writing mode</a> is instead taken
+  from the values of 'writing-mode' and 'direction' on the first such child element
+  instead of taken from the root element.
+  Note that this does not affect the values of 'writing-mode' or 'direction'
+  on the root element itself.
+
+<h3 id="icb">
+Propagation to the Initial Containing Block</h3>
+
+  <p>The <a>principal writing mode</a> is propagated to
+  the <a>initial containing block</a>
+  and to the viewport,
+  thereby affecting the layout of the root element
+  and the scrolling direction of the viewport.
+
+  Issue: Does it also propagate to @page boxes?
 
 <h3 id="page-direction">
 Page Flow: the page progression direction</h2>
 
-  <p>The <dfn export>principal writing mode</dfn> of the
-    document is determined by the 'writing-mode' and 'direction' values
-    specified on the root element. This writing mode is used, for example,
-    to determine the default page progression direction. (See [[CSS3PAGE]].)
-
-  <p>In paged media CSS2.1 classifies all pages as either left or right pages.
-    The page progression direction, which determines whether the left or right
-    page in a spread is first in the flow and whether the first page is by
-    default a left or right page, depends on the writing direction as follows:
+  <p>In paged media CSS classifies all pages as either left or right pages.
+  The <a>page progression</a> direction (see [[CSS3PAGE]]),
+  which determines whether the left or right page in a spread is first in the flow
+  and whether the first page is by default a left or right page,
+  depends on the <a>principal writing mode</a> as follows:
 
-  <ol>
-    <li>If the root element's 'writing-mode' is ''vertical-rl'',
-      the page progression is right-to-left.
-    <li>Otherwise if the root element's 'writing-mode' is ''vertical-lr'',
-      the page progression is left-to-right.
-    <li>Otherwise if the root element's 'direction' is ''rtl'',
-      the page progression is right-to-left.
-    <li>Otherwise
-      the page progression is left-to-right.
-  </ol>
+  <table class="data">
+    <thead>
+      <tr>
+        <th><a>principal writing mode</a>
+        <th><a>page progression</a>
+    <tbody>
+      <tr>
+        <td>''horizontal-tb'' and ''ltr''
+        <td>left-to-right
+      <tr>
+        <td>''horizontal-tb'' and ''rtl''
+        <td>right-to-left
+      <tr>
+        <td>''vertical-rl'' or ''sideways-rl''
+        <td>right-to-left
+      <tr>
+        <td>''vertical-lr'' or ''sideways-lr''
+        <td>left-to-right
+  </table>
 
-  <p>(Unless otherwise overridden, the first page of a document begins on the
-    second half of a spread, e.g. on the right page in a left-to-right page
-    progression.)
+  Note: Unless otherwise overridden,
+  the first page of a document begins on the second half of a spread,
+  e.g. on the right page in a left-to-right page progression.
 
 <h2 id="text-combine">
 Glyph Composition</h2>
@@ -2613,8 +2629,8 @@ Full-width Characters</h5>
       Removed statements about ''run-in'' boxes, since this value was removed from CSS2.1 and has a substantially different model in CSS Display Level 3.
       Replaced with a generalized statement about all display types, using new [[!CSS-DISPLAY-3]] terminology.
     <li>
-      Changed to <a href="#propagate-to-icb">propagate the computed values</a> of the 'writing-mode' and the 'direction' property
-      from body child element to the root element.
+      Changed to <a href="#principal-writing-mode">propagate the computed values</a> of the 'writing-mode' and the 'direction' property
+      from body child element to the initial containing block.
     <li>
       Changed the 'caption-side' property to <a href="#logical-direction-layout">flow-relative mappings</a>.
     <li>