The HTML AAM currently maps body to the ARIA document role. But some accessibility APIs expect different mappings. For instance, in macOS the accessible object corresponding to body has a role of AXWebArea but the ARIA document role is an AXGroup (with a subrole of AXDocument). Similarly, in ATK, the accessible object corresponding to body should[*] be exposed as ATK_ROLE_DOCUMENT_WEB, with ATK_ROLE_DOCUMENT_FRAME being used for the document role.

[*] Gecko currently fails to do this, but I plan to provide a patch to fix that. WebKitGtk does it correctly.