Define rules for transitioning 'visibility' so that transitions where one of the values is 'collapse' have defined behavior.

This is largely as proposed in
http://lists.w3.org/Archives/Public/www-style/2012Feb/1083.html and
resolved in the 2012 March 7 teleconference (minutes at
http://lists.w3.org/Archives/Public/www-style/2012Mar/0195.html ).

However, I realized we didn't discuss the issue of what happens when the
result of the timing functions it outside of (0, 1).  I addressed this
as described in
http://lists.w3.org/Archives/Public/www-style/2012Mar/0597.html

Author: dbaron

css3-transitions/Overview.html

@@ -1141,9 +1141,15 @@ <h2 id=animation-of-property-types-><span class=secno>6. </span> Animation
    <li> <strong>rectangle</strong>: interpolated via the x, y, width and
     height components (treating each as a number).
 
-   <li> <strong>visibility</strong>: interpolated via a discrete step. The
-    interpolation happens in real number space between 0 and 1, where 0 is
-    "hidden" and all other values are "visible".
+   <li> <strong>visibility</strong>: if one of the values is &lsquo;<code
+    class=property>visible</code>&rsquo;, interpolated as a discrete step
+    where values of the timing function between 0 and 1 map to &lsquo;<code
+    class=property>visible</code>&rsquo; and other values of the timing
+    function (which occur only at the start/end of the transition or as a
+    result of &lsquo;<code class=css>cubic-bezier()</code>&rsquo; functions
+    with Y values outside of [0, 1]) map to the closer endpoint; if neither
+    value is &lsquo;<code class=property>visible</code>&rsquo; then not
+    interpolable.
 
    <li> <strong>shadow</strong>: interpolated via the color, x, y and blur
     components (treating them as color and numbers where appropriate). In the

css3-transitions/Overview.src.html

@@ -1124,9 +1124,14 @@ <h2 id="animation-of-property-types-">
           width and height components (treating each as a number).
         </li>
         <li>
-          <strong>visibility</strong>: interpolated via a discrete
-          step. The interpolation happens in real number space between 0 and 1,
-          where 0 is "hidden" and all other values are "visible".
+          <strong>visibility</strong>: if one of the values is
+          'visible', interpolated as a discrete step where values of the
+          timing function between 0 and 1 map to 'visible' and other
+          values of the timing function (which occur only at the
+          start/end of the transition or as a result of 'cubic-bezier()'
+          functions with Y values outside of [0, 1]) map to the closer
+          endpoint; if neither value is 'visible' then not interpolable.
+          </span>
         </li>
         <li>
           <strong>shadow</strong>: interpolated via the color, x, y