Added that the available width of a block with a different orientation

as its parent is taken from the viewport.

Author: bert-github

css3-box/Overview.src.html

@@ -1525,13 +1525,11 @@ <h2>The padding properties</h2>
 
 
 
-<h2>Margins</h2>
+<h2>The margin properties</h3>
 
 <p>Margins in CSS serve to add both horizontal and vertical space
 between boxes.
 
-<h3>The margin properties</h3>
-
 <table class="propdef">
   <tr><th>Name: <td><dfn>margin-top</dfn>, <dfn>margin-right</dfn>,
     <dfn>margin-bottom</dfn>, <dfn>margin-left</dfn>
@@ -2763,15 +2761,32 @@ <h3 id=blockwidth>Block-level, non-replaced elements
   </div>
 </div>
 
-<p>With respect to the <span>inline dimension,</span> the following
-constraint must hold among the used values of the given properties.
+<p>With respect to the <span>inline dimension,</span> one of the
+following constraints must hold among the used values of the given
+properties.
+
+<ul id=width-constraints>
+  <li>If the element has the same orientation as its containing block
+  (i.e., both are <em>horizontal</em> or both are <em>vertical</em>):
 
-  <blockquote id=width-constraints>
+  <blockquote>
   <p>start margin + start border + start padding + inline dimension +
   end padding + end border + end margin = inline dimension of
   containing block
   </blockquote>
 
+  <li>Otherwise the constraint is relative to the height or width of
+  the <em>initial containing block,</em> depending on whether the
+  element is <span>horizontal</span> or <span>vertical,</span>
+  respectively:
+
+  <blockquote>
+  <p>start margin + start border + start padding + inline dimension +
+  end padding + end border + end margin = width or height of
+  <em>initial containing block</em>
+  </blockquote>
+</ul>
+
 <p>The used values of the above properties, except for any values that
 are ''auto'', are calculated from the computed values, evaluating
 percentages. If this makes the inline dimension negative, then set