Allow ignoring position or dimensions in set window rect

Previously it was unclear if it was acceptable to return without error
from set window rect if the implementation is unable to set some of
the properties.

This updates the spec so that:

* It's unambiguously OK to run the command even if you don't support
some aspects of the functionality on the current platform /
configuration (you are expected to continue to return an error if you
don't support any aspect of the functionality).

* It's clear that both position and dimension properties are best
effort, and even if you are able to adjust them there's no guarantee
that you get the exact requested values.

* It's clear what to do if you get just an x coordinate but not y, or
just width but not height, or vice-versa. Previously this was undefined.

index.html

@@ -4044,25 +4044,23 @@ <h4><dfn>Set Window Rect</dfn></h4>
 <ol>
  <li><p>Let <var>width</var> be the result of
   <a>getting a property</a> named "<code>width</code>"
-  from <var>parameters</var>, else let it be <a><code>null</code></a>.
+  from <var>parameters</var></a>.
 
  <li><p>Let <var>height</var> be the result of
   <a>getting a property</a> named "<code>height</code>"
-  from <var>parameters</var>, else let it be <a><code>null</code></a>.
+  from <var>parameters</var>.
 
  <li><p>Let <var>x</var> be the result of <a>getting a property</a>
-  named "<code>x</code>" from <var>parameters</var>,
-  else let it be <a><code>null</code></a>.
+  named "<code>x</code>" from <var>parameters</var>.
 
  <li><p>Let <var>y</var> be the result of <a>getting a property</a>
-  named "<code>y</code>" from <var>parameters</var>,
-  else let it be <a><code>null</code></a>.
+  named "<code>y</code>" from <var>parameters</var>.
 
- <li><p>If <var>width</var> or <var>height</var> is neither <a><code>null</code></a>
+ <li><p>If <var>width</var> or <var>height</var> is neither <a>undefined</a>
   nor a <a>Number</a> from 0 to 2<sup>31</sup> − 1,
   return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
 
- <li><p>If <var>x</var> or <var>y</var> is neither <a><code>null</code></a>
+ <li><p>If <var>x</var> or <var>y</var> is neither <a>undefined</a>
   nor a <a>Number</a> from −(2<sup>31</sup>) to 2<sup>31</sup> − 1,
   return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
 
@@ -4071,6 +4069,11 @@ <h4><dfn>Set Window Rect</dfn></h4>
   <var>session</var>&apos;s <a>current top-level browsing context</a> for any reason,
   return <a>error</a> with <a>error code</a> <a>unsupported operation</a>.
 
+  <p class="note">In case the <a>Set Window Rect</a> command is
+  partially supported (i.e. some combinations of arguments are
+  supported but not others), the implmentation is expected to continue
+  with the remaining steps.
+
  <li><p>If <var>session</var>&apos;s <a>current top-level browsing
   context</a> is <a>no longer open</a>, return <a>error</a>
   with <a>error code</a> <a>no such window</a>.
@@ -4082,21 +4085,25 @@ <h4><dfn>Set Window Rect</dfn></h4>
 
  <li><p><a>Restore the window</a>.
 
- <li><p>If <var>width</var> and <var>height</var> are not <a><code>null</code></a>:
+ <li><p>Let <var>window</var> be the operating system window containing
+     <var>session</var>&apos;s <a>current top-level browsing context</a>
+
+ <li><p>If the implementation is able to set the dimensions
+ of <var>window</var>:
 
   <ol>
-   <li><p>Set the width, in <a>CSS pixels</a>,
-    of the operating system window containing
-    <var>session</var>&apos;s <a>current top-level browsing context</a>,
-    including any <a>browser chrome</a> and externally drawn window decorations
+   <li><p>If <var>width</var> is not <a>undefined</a>, set the width,
+    in <a>CSS pixels</a>, of <var>window</var>, including
+    any <a>browser chrome</a> and externally drawn window decorations,
     to a value that is as close as possible to <var>width</var>.
 
-  <li><p>Set the height, in <a>CSS pixels</a>,
-   of the operating system window containing
-   <var>session</var>&apos;s <a>current top-level browsing context</a>,
-   including any <a>browser chrome</a> and externally drawn window decorations
+  <li><p>If <var>height</var> is not <a>undefined</a>, set the height,
+   in <a>CSS pixels</a>, of <var>window</var>, including
+   any <a>browser chrome</a> and externally drawn window decorations,
    to a value that is as close as possible to <var>height</var>.
 
+  </ol>
+
    <aside class=note>
     <p>The specification does not guarantee
      that the resulting window size will exactly match that which was requested.
@@ -4116,35 +4123,37 @@ <h4><dfn>Set Window Rect</dfn></h4>
      of <a>outerHeight</a> should be as close as possible
      to <var>height</var>.
    </aside>
-  </ol>
 
- <li><p>If <var>x</var> and <var>y</var> are not <a><code>null</code></a>:
+ <li><p>If the implementation is able to set the position of <var>window</var>:
 
   <ol>
-   <li><p>Run the implementation-specific steps to set the position of
-    the operating system level window
-    containing <var>session</var>&apos;s <a>current top-level browsing
-    context</a> to the position given by the <var>x</var>
-    and <var>y</var> coordinates.
-
-    <aside class=note>
-     <p>Note that this step is similar to calling the <a>moveTo(x,
-      y)</a> method on the <a><code>WindowProxy</code></a> object
-      associated with <var>session</var>&apos;s <a>current top-level browsing
-      context</a>, but without the
-      <a href=https://developer.mozilla.org/en-US/docs/Web/API/Window/moveTo>security
-      restrictions</a> that you
-
-     <ol type=a>
-      <li>cannot move a window or tab
-       that was not created by <code>window.open</code>.
-
-      <li>cannot move a window or tab
-       when it is in a window with more than one tab.
-     </ol>
-   </aside>
+   <li><p>If <var>x</var> is not <a>undefined</a>, run specific
+    steps set the x-coordinate of the left edge of <var>window</var>
+    to a value that is as close as possible to <var>x</var>.
+
+   <li><p>If <var>y</var> is not <a>undefined</a>, run specific
+    steps set the y-coordinate of the top edge of <var>window</var> to
+    a value that is as close as possible to <var>y</var>.
   </ol>
 
+  <aside class=note>
+    <p>The specification does not guarantee
+     that the resulting window position will match that which was requested.
+
+   <p>This step is similar to calling the <a>moveTo(x,
+    y)</a> method on the <a><code>WindowProxy</code></a> object
+    associated with <var>session</var>&apos;s <a>current top-level browsing
+    context</a>, but without the
+    <a href=https://developer.mozilla.org/en-US/docs/Web/API/Window/moveTo>security
+    restrictions</a> that you
+
+   <ol type=a>
+    <li>cannot move a window or tab that was not created by <code>window.open</code>.
+
+    <li>cannot move a window or tab when it is in a window with more than one tab.
+   </ol>
+  </aside>
+
  <li><p>Return <a>success</a> with data set to the <a>WindowRect
   object</a> for the <var>session</var>&apos;s <a>current top-level browsing
   context</a>.