Cursors: mark <cursor> element deprecated, update other prose

This is consistent with CSS fully supporting custom cursors, and with HTML specifically disallowing cursor references to cursor elements embedded in an HTML file.

Also add clarifying text to the Linking chapter on how to process URLs when the valid target can be either a specific element or an image file (i.e., cursor and mask properties).

Closes #174

master/interact.html

@@ -792,26 +792,30 @@ <h2 id="Cursors">Cursors</h2>
 
 <ul>
   <li>Standard built-in cursors</li>
-  <li>Platform-specific custom cursors</li>
-  <li>Platform-independent custom cursors</li>
+  <li>Custom cursors</li>
 </ul>
 
 <p>The <a>'cursor property'</a> property is used to
 specify which cursor to use. The <a>'cursor property'</a> property can be used to
 reference standard built-in cursors by specifying a keyword
 such as <em>crosshair</em> or a custom cursor. Custom cursors
-are referenced via a <a>&lt;url&gt;</a> and can point to either an
-external resource such as a platform-specific cursor file or to
-a <a>'cursor element'</a> element, which can be
+are defined by referencing an image that can be
 used to define a platform-independent cursor.</p>
+
+<p>When defining a platform-independent cursor file,
+  either the SVG or the PNG format are recommended,
+  because they support alpha transparency to define a cursor shape.
+  Cursor images ideally contain at least two colors so
+  that the cursor can be visible over most backgrounds.</p>
+
 
 <div class="ready-for-wider-review">
 <h3 id="CursorProperty">The effect of the <span class="property">'cursor'</span> property</h3>
 
 <p class="note">See the CSS Basic User Interface Module Level 3 specification
-for the definition of <a>'cursor property'</a>. [<a href="refs.html#ref-CSS3UI">CSS3UI</a>]</p>
+for the definition of <a href="https://www.w3.org/TR/css-ui-3/#cursor">cursor</a>. [<a href="refs.html#ref-CSS3UI">CSS3UI</a>]</p>
 
-<p>SVG uses the <a>'cursor property'</a> property to specify the type of cursor to
+<p>The <a>'cursor property'</a> property specifies the type of cursor to
 be displayed for the pointing device when it is over a region of an
 element that is sensitive to pointer events, according to the value of
 the <a>'pointer-events'</a> property.  SVG extends the definition of
@@ -829,29 +833,17 @@ <h3 id="CursorProperty">The effect of the <span class="property">'cursor'</span>
 </div>
 
 <h3 id="CursorElement">The <span class="element-name">'cursor'</span> element</h3>
+
+<p>The cursor element is deprecated, as the standard CSS <a>'cursor property'</a>
+  now supports custom image references.
+  User agents are required to support it in interactive SVG documents only.
+  </p>
 
 <edit:with element='cursor'>
 
 <p>The <a>'cursor element'</a> element can
-be used to define a platform-independent custom cursor. A
-recommended approach for defining a platform-independent custom
-cursor is to create a PNG image [<a href="refs.html#ref-PNG">PNG</a>]
-and define a <a>'cursor element'</a> element
-that references the PNG image and identifies the exact position
-within the image which is the pointer position (i.e., the hot spot).</p>
-
-<p>The PNG format is recommended because it supports the
-ability to define a transparency mask via an alpha channel. If
-a different image format is used, this format should support
-the definition of a transparency mask (two options: provide an
-explicit alpha channel or use a particular pixel color to
-indicate transparency). If the transparency mask can be
-determined, the mask defines the shape of the cursor;
-otherwise, the cursor is an opaque rectangle. Typically, the
-other pixel information (e.g., the R, G and B channels) defines
-the colors for those parts of the cursor which are not masked
-out. Note that cursors usually contain at least two colors so
-that the cursor can be visible over most backgrounds.</p>
+be used to define an image for a platform-independent custom cursor,
+and the offset from the image dimensions to the active cursor point.</p>
 
 <edit:elementsummary name='cursor'/>
 
@@ -877,6 +869,8 @@ <h3 id="CursorElement">The <span class="element-name">'cursor'</span> element</h
   <dd>
     The x- and y-coordinates of the position in the cursor's coordinate system
     which represents the precise position that is being pointed to.
+    These coordinates apply if coordinates are not specified 
+    when the <a>'cursor element'</a> is referenced.
   </dd>
   <dt>
     <table class="attrdef def">

master/linking.html

@@ -670,13 +670,22 @@ <h4 id="processingURL-validity">Valid URL targets</h4>
   <!--
   <li>the <a href="color.html#ColorProfileSrcProperty">'src'</a> descriptor on an @color-profile definition must reference an ICC profile resource</li>
   -->
-  <li>the <a>'cursor property'</a> property must reference a resource that can provide an image for the cursor graphic</li>
+  <li>the <a>'cursor property'</a> property must reference either a <a>'cursor element'</a> element or a document that can be processed as an image</li>
   <li>the <a>'fill'</a> property (see <a href="painting.html#SpecifyingPaint">Specifying paint</a> for reference rules)</li>
   <li>the <a>'marker property'</a>, <a>'marker-start'</a>, <a>'marker-mid'</a> and <a>'marker-end'</a> properties must reference a <a>'marker element'</a> element.</li>
   <li>the <a>'stroke'</a> property (see <a href="painting.html#SpecifyingPaint">Specifying paint</a> for reference rules)</li>
 </ul>
 
-<p>If the resolved target element type (or document type) is not allowed for the URL reference,
+<p>
+  For references that allow either a reference to a target element, or to an image file
+  (such as the <a>'cursor property'</a> and <a>'mask property'</a>),
+  the user agent must identify the target element and determine whether it is a valid target.
+  If the resolved target element is not an allowed element type,
+  the referenced resource is the entire document file;
+  the target fragment is used in processing that file as with any other image.
+  </p>
+
+<p>In all other cases, if the resolved target element type (or document type) is not allowed for the URL reference,
   it is an <a>invalid reference</a>.
 </p>