[css-anchor-position] Add some a11y guidance. w3c#9356

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1161,6 +1161,48 @@ were ''visibility: hidden'',
 regardless of what their actual 'visibility' value is.
 
 
+Accessibility Implications {#accessibility}
+--------------------------
+
+It's important to remember that Anchor Positioning
+does not automatically establish any semantic relationship
+between a positioned element
+and any of its anchors,
+because it can be used in many different ways.
+Authors must not rely solely on a <em>visual</em> connection
+implied by the positioning
+to link elements together semantically;
+without additional help,
+the elements often have <em>no</em> meaningful DOM relationship,
+making them difficult or impossible to use in non-visual user agents,
+like screen readers.
+
+Many features on the web platform,
+both existing and upcoming,
+allow establishing such connections explicitly,
+so that non-visual user agents can also benefit.
+
+For example, the Popover API in HTML
+automatically links the invoker button
+to the popover element,
+including automatically adjusting tabbing order;
+it <em>also</em> establishes the invoker button
+as the [=implicit anchor element=] for the popover,
+making it easy to use Anchor Positioning as well.
+
+In more general cases,
+ARIA features such as
+the <a href="https://w3c.github.io/aria/#aria-details"><code>aria-details</code></a>
+or <a href="https://w3c.github.io/aria/#aria-describedby"><code>aria-describedby</code></a> attributes
+on an anchor element
+can provide this information
+in a slightly more manual fashion;
+in concert with the <{html-global/role}> attribute on the positioned element,
+non-visual user agents
+can tell their users about the relationship between the elements
+and let them automatically navigate between them.
+
+
 
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