Add normative text on calculating aria-level, aria-posinset, and aria-setsize

accname/index.html

@@ -628,7 +628,8 @@ <h4>Computation steps</h4>
                 <li id="comp_host_language_label">
                   <span id="step2E"><!-- Don't link to this legacy numbered ID. --></span><em>Host Language Label:</em> Otherwise, if the <code>current node</code>'s native markup provides an
                   [=attribute=] (e.g. <code>alt</code>) or [=element=] (e.g. HTML <code>label</code> or SVG <code>title</code>) that defines a text alternative, return that alternative in the form of
-                  a <code>flat string</code> as defined by the host language, unless the <code>current node</code> is exposed as presentational (<code>role="presentation"</code> or <code>role="none"</code>).
+                  a <code>flat string</code> as defined by the host language, unless the <code>current node</code> is exposed as presentational (<code>role="presentation"</code> or
+                  <code>role="none"</code>).
                   <div class="note">
                     See <a href="https://www.w3.org/TR/html-aam-1.0/#accessible-name-and-description-computation">HTML-AAM</a>,
                     <a href="https://www.w3.org/TR/svg-aam-1.0/#mapping_additional_nd">SVG-AAM</a>, or other host language documentation for more information on markup that defines a text alternative.

index.html

@@ -14398,10 +14398,10 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
               container, restricting this to leaf nodes ensures that there is a single way for <a>assistive technologies</a> to use the attribute.
             </p>
             <p>
-              If the <abbr title="Document Object Model">DOM</abbr> ancestry accurately represents the level, the <a>user agent</a> can calculate the level of an item from the document structure. This
-              attribute can be used to provide an explicit indication of the level when that is not possible to calculate from the document structure or the <pref>aria-owns</pref> attribute. User
-              agent support for automatic calculation of level might vary; authors SHOULD test with [=user agents=] and assistive technologies to determine whether this attribute is needed. If the
-              author intends for the user agent to calculate the level, the author SHOULD omit this attribute.
+              If the <abbr title="Document Object Model">DOM</abbr> ancestry accurately represents the level, the <a>user agent</a> can calculate the level of an item from the document structure as
+              defined in the <a href="#tree_groups">Calculating Group Position</a> section. This attribute can be used to provide an explicit indication of the level when that is not possible to
+              calculate from the document structure or the <pref>aria-owns</pref> attribute. User agent support for automatic calculation of level might vary; authors SHOULD test with [=user agents=]
+              and assistive technologies to determine whether this attribute is needed. If the author intends for the user agent to calculate the level, the author SHOULD omit this attribute.
             </p>
             <p class="note">
               In the case of a <rref>treegrid</rref>, <pref>aria-level</pref> is supported on elements with the role <rref>row</rref>, not elements with role <rref>gridcell</rref>. At first glance,
@@ -14948,6 +14948,7 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
               When specifying <code>aria-posinset</code> on a <rref>menuitem</rref>, <rref>menuitemcheckbox</rref>, or <rref>menuitemradio</rref>, authors SHOULD set the value of
               <code>aria-posinset</code> with respect to the total number of items in the <rref>menu</rref>, excluding any separators.
             </p>
+            <p>If <code>aria-posinset</code> is not specified by the author, user agents MUST compute it as defined in the <a href="#tree_groups">Calculating Group Position</a> section.</p>
           </div>
           <table class="def">
             <caption>
@@ -15748,6 +15749,7 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
               When specifying <code>aria-setsize</code> on a <rref>menuitem</rref>, <rref>menuitemcheckbox</rref>, or <rref>menuitemradio</rref>, authors SHOULD set the value of
               <code>aria-setsize</code> based on the total number of items in the <rref>menu</rref>, excluding any separators.
             </p>
+            <p>If <code>aria-posinset</code> is not specified by the author, user agents MUST compute it as defined in the <a href="#tree_groups">Calculating Group Position</a> section.</p>
             <p>The following example shows items 5 through 8 in a set of 16.</p>
             <pre class="example highlight">
 &lt;h2 id="label_fruit"&gt; Available Fruit &lt;/h2&gt;
@@ -16230,6 +16232,49 @@ <h2>Relationships in the Accessibility Tree</h2>
 &lt;/div&gt;
           </pre
         >
+        <section id="tree_groups">
+          <h3>Calculating Group Position in the Accessibility Tree</h3>
+          <p>
+            When <pref>aria-level</pref>, <pref>aria-posinset</pref>, or <pref>aria-setsize</pref> are defined on supported roles, they allow authors to override the default group position and level
+            calculation. When those attributes are not specified by the author, user agents MUST compute them following the rules in this section.
+          </p>
+          <p>
+            If <pref>aria-level</pref> is not specified on <rref>comment</rref>, <rref>row</rref> within a <rref>treegrid</rref>, or <rref>treeitem</rref>, user agents MUST compute the level as
+            follows:
+          </p>
+          <ul>
+            <li>
+              Walk up the accessibility tree, and count the number of accessibility ancestors with the relevant grouping role. Those roles are as follows:
+              <ul>
+                <li>For <rref>comment</rref>, count the number of ancestors with role <rref>comment</rref>.</li>
+                <li>For <rref>row</rref> within a <rref>treegrid</rref>, count the number of ancestors within the same <rref>treegrid</rref> with role <rref>rowgroup</rref>.</li>
+                <li>For <rref>treeitem</rref>, count the number of ancestors within the same <rref>tree</rref> with role <rref>group</rref>.</li>
+              </ul>
+            </li>
+            <li>Return 1 + the number of ancestors with the relevant grouping role.</li>
+          </ul>
+          <p>
+            If <pref>aria-posinset</pref> or <pref>aria-setsize</pref> is not specified on any role the supports it with the exception of <rref>article</rref>, user agents MUST compute them as
+            follows:
+          </p>
+          <ul>
+            <li>
+              For <rref>treeitem</rref> and <rref>comment</rref>: walk the accessibility tree backward and forward until the explicit or computed level becomes less than the current item's level.
+              Count items only if they are at the same level as the current item.
+            </li>
+            <li>For all other supported roles: process the accessibility parent, counting all accessibility children that have the same role.</li>
+            <li>For <pref>aria-posinset</pref>, return 1 + the number of group items before the current item in the accessibility tree.</li>
+            <li>For <pref>aria-setsize</pref>, return the total number of group items, including the current item.</li>
+          </ul>
+          <p>
+            If the author provides one or more of <pref>aria-setsize</pref> and <pref>aria-posinset</pref>, it is the author's responsibility to supply them for all elements in the set. User agent
+            correction of missing values in this case is not defined.
+          </p>
+          <p>
+            <pref>aria-level</pref>, <pref>aria-posinset</pref>, and <pref>aria-setsize</pref> are all 1-based. When the property is not present or is "0", it indicates the property is not computed or
+            not supported. If any of these properties are specified by the author as either "0" or a negative number, user agents SHOULD use "1" instead.
+          </p>
+        </section>
       </section>
     </section>
     <section class="normative" id="host_languages">