Currently GTK 4 has no support for a11y. We should start by documenting what would be required to make this work.
What we currently have is a an a11y tree in the UI process built by GTK until WebKitWebView. The web view has is own accessible derived from AtkSocket. In the web process we have another a11y tree, coming from the DOM, so not associated to any GTK widget and it's built by WebKit using ATK API. The root of web process a11y tree is an AtkPlug. So, we use the ATK Socket/Plug API to connect the UI process tree with the web process one.
In GTK4 most of the a11y APIs are private, internally implemented by GtkWidgets, so our WebKitWebView has a GtkAtContext by default. We need a way to override the WebKitWebView a11y implementation, or at least some of the methods. For example GetChildren returns the references to the widgets that are children of WebKitWebView, but we need to include there the root object of the DOM a11y tree. Ideally, I think it should be enough if we could override GetChildren and GetChildAtIndex methods and ChildCount property somehow. Then we would need a way to get the WebKitWebView a11y object path, so that the root of DOM a11y tree can set the Parent property accordingly.
Is this fixed?
No, we still need a way to override the WebKitWebView a11y implementation. See comment #2
This depends on: https://gitlab.gnome.org/GNOME/gtk/-/issues/3597
Looks like https://gitlab.gnome.org/GNOME/gtk/-/issues/5162 will provide a path forward here.