Review known issues in Oracle Application Express in the following documents.
VPAT
VPATs for Oracle Database related products — VPAT for Oracle Application Express. Click the link, then find the relevant version of Application Express.
Affecting the Development Environment
Affecting Websheet Applications
Affecting Database Applications
Affecting Documentation
Release Notes
"Accessibility Issues" in the Oracle Application Express Release Notes lists less severe issues with corresponding bug number, usually where something can be worked around or where the impact is minimal.